During the incubation in darkness of positive photoblastic seeds processes occur that increase the effectiveness of light, in particular of red light (R), in causing germination of these seeds. There is evidence in the literature that in the preirradiation (dark) period the primary site of reaction of the far-red absorbing form of phytochrome (Pfr) or the substance(s) with which Pfr reacts are formed (Roller et al. 1964, Karssen 1967, 1970). We investigated for lettuce seeds the possibility that this preparatory reaction is directly under control of Pfr, a possibility that was not considered by Roller et al. for Artemisia monosperma seeds nor by Rarssen for Chenopodium album seeds. Materials and methods were according to Blaauw-Jansen & Sewmar (1985). Dormancy was induced in Lactuca sativa L. ‘May Queen’ seeds by imbibition under continuous FR. Germination was induced by irradiation with a high or a low fluence of R, either immediately after the imbibition period or after a dark period of 24 h. The exposure times were 60 s (0.2 W m“2) or 40 s (0.03 W m“2). After the exposure to R the seeds were returned to darkness and every 30 min 2 dishes were removed for counting without illuminating the remaining dishes. The results of the experiments, expressed in percentages germination, are means of 2 dishes. In order to restrict the periodically counting of the germinated seeds within a reasonable time of the day the dishes for each experiment were divided into 3 sets. The start of the imbibition of each set was timed at different hours with the effect that counting during 8 h gave the course of germination during 26 to 40 h after irradiation. This implies that each curve representing the course of germination is build up out of 3 overlapping parts. As the overlapping parts were in good agreement it was thought unnecessary to repeat the experiments.