During a recorded high seabird-mortality in the North Sea in the winter 1982/83, large numbers of dead seabirds found on the Netherlands coast were collected and dissected (table 1). The resulting paper attempts to construct a model for registration and interpretation of internal anatomical characters. The objectives of the autopsies were twofold: One was to assess the sex and age composition within groups of victims. Sex was determined by inspection of the reproductive organs, which also indicates reproductive capacity, an age-related factor. Another age determining character is the bursa Fabricii, an internal organ involved in immunity. The bursa is strongly developed in juveniles, but gets smaller with increasing age and eventually disappears. Figure 1 is a summary of age-related characters in reproductive organs and bursa, and it supplies a method for describing these characters in indices. For comparative studies it has to be stressed that testis- and follicle/oviduct-index are not just related to age, but also to annual fluctuations in sexual activity. The second objective was to try to determine possible factors contributing to death. Because of certain limitations only emaciation and condition of internal organs were studied. Although the extent of emaciation and morphological defect of organs rarely gives the direct causes leading to death, it may have a monitor-function for (changes in) involvement of certain factors. Figure 2 summarizes the details on the internal parts studied, and the codes for registration of their condition. Due to its doubtless part in mortality, external contamination of feathers with oil (table 2) is included in this paper. Major results are shown in tables 3-6. All the codes used are explained in figures 1 and 2; generally low figures represent unhealthy, high figures healthy conditions. Emaciation or condition of organs were only calculated from carcasses that had not been previously subject to attempts of rehabilitation. An important result in sex-composition was noted in the high preponderance of female victims in the Kittiwake (78%) and the Fulmar (71%) as compared to other species. There was a rather marked difference between percentages of adult females determined using the bursa-system (no bursa = adult) and those by oviduct-shape (oviductcodes 3-4 = adult). Further data on such differences are discussed in detail below. The large majority of birds was completely emaciated at death. Many birds suffered from minor oil-contaminations in their plumage, whereas only 10 to 20% of the specimens was not oiled. In most species over 50% of the birds is considered to have ingested oil. Large percentages of the birds were heavily affected in gut and lungs. Plastic ingestion may be an important factor affecting some species. In the Kittiwake 37.5% and in the Fulmar 86.2% of the victims had ingested plastic items. A major part of the discussion is focussed on development of reproductive organs, with reference to various systems (bursa, morphology) for determining adults. A more detailed discussion on nonadults is given in the paper of De Wijs in this issue. Methods in determining sexual maturity are discussed for Razorbills (figures 3 & 4), Guillemots (figures 5 & 6), Kittiwake (figure 7), and Fulmar (figure 8). It is argued that female Razorbills, Guillemots, and Fulmars cannot be considered to be sexually mature when the foll./ovid.-index is smaller than 3, or in Kittiwake when smaller than 1.5. In male Razorbills and Guillemots a testis-index <25 is considered to represent sexual immaturity. The index value for adults varies in different species: values used herein are subjective and should be confirmed with further studies. In table 7 percentages of adult birds determined by lack of bursa, versus adult percentages determined using gonad-condition, are represented (differences with table 3b are caused by including males in some species and by including follicle-size in the judgement on female reproductive organs). Table 7 indicates that the use of bursacondition alone would give biased results, as clearly seen by the smaller number of gonad-mature birds. The difference is strongest in the Fulmar, which may be explained by its late age of first breeding. It appears from the data that most birds have diminished bursa’s after a few years (2 to 3 years in the Kittiwake and probably in the Razorbill), which apparently means that the later a species starts breeding, the greater is the error in the bursa-method (or morphological method) of ageing adults. Further studies are needed to determine more precisely the “bursa-disappearing-time” in different species. In Kittiwake, Fulmar and Razorbill the percentage of sexually mature birds is virtually the same when calculations are restricted to specimens without a bursa, or when birds with a bursa are included. However, in the Guillemot the percentage of sexually mature birds changes from 32% into 40% when including birds with a bursa in the calculations. The combination of having a bursa as well as rather developed reproductive organs is rather common in the Guillemot (see figures 5 and 6). Several explanations of such a phenomenon are considered too speculative to be discussed in detail. Concerning aspects of mortality it is striking that very large numbers of birds have minor oil-patches on the plumage: almost all such birds die only after moderate to strong emaciation (table 8). Birds heavily contaminated with oil generally die quickly, but their number is rather small. Many birds therefore probably are weakened, not di- rectly killed. Because of this weakening, birds become more vulnerable to other negative factors like chemical pollutants (e.g. organochiorines stored in the fat) and ingested plastics. Many birds ingest oil by preening or eating from oil-contaminated objects (table 9 shows that our opinion on the oil-condition of the stomach is not an artefact caused by reversed peristaltic movements of affected guts). There is evidence chat oil-ingestion is more common in birds with lesser amounts of oiled plumage. Over 30% of the specimens not having oil on their feathers probably has ingested oil. An example of the processes that affect weakened birds is shown in table 10. Parasitical worms in the stomach occur more frequently and in larger numbers in emaciated birds: these parasites contribute to further weakening once the process has started. Responsability for recent bird mortalities cannot be simply ascertained by counting the number of oiled birds. Many other factors contribute to mortality, even if there is little doubt that oil is the main “trigger-factor” in the weakening process. This study does not try to judge the responsability of all factors involved. It can only be hoped that important changes in recent mortality factors will be monitored by changes in the observed pattern of emaciation and condition of organs. Birds that died during rehabilitation in spite of food-intake showed similar emaciation as beached birds (table II). In spite of a healthier appearance of stomach and gut the actual uptake of nutrients in the gut seems to be absent. Ingested oil is thought to be an important factor in this malfunction of the gut. In rehabilitation also a lack of improvement of lung condition is noted. In conclusion dissections are an important aspect of studies on seabirdmortality: it supplies information on sex and age composition and helps in monitoring important changes in the processes that contribute to mortality. This paper offers an example of the potential use of dissections and hopes to stimulate further work.