Analysing molecular processes and cytological changes coinciding with induction of embryogenesis in microspores and pollen of Brassica napus c.v. Topas, attention was paid to the induction and dynamics of DNA synthesis in nuclei of microspores and pollen and to changes in the microtubular and microfilamental arrangements during the first days of culture. The patterning of the microfilamental network, observed after rhodamine-phalloidin staining of whole cells, does not show prominent changes. The microtubular network, visualized immunocytochemically on sectioned material, however, exhibited a number of changes that directly relate to changed division patterns: (i) symmetrical divisions in microspores are preceded by a change of spindle orientation; (ii) symmetrical divisions in microspores are induced after a shift in nuclear position caused by the disruption or disturbed formation of cytoplasmic microtubules; and (iii) symmetrical divisions are induced in vegetative cells when generative cells remain attached to the pollen wall which is probably caused by the disappearance of cytoplasmic microtubules. (See Hause, B. et al. (1993): Cell Biol Intern. 17: 153-168.) DNA synthesis was visualized immunocytochemically on sectioned material after pulse or continuous labelling of isolated microspores and pollen cultured under embryogenic (32°C) and non-embryogenic (18°C) conditions. DNA contents of nuclei were determined microspectrophotometrically. (i) Microspores do show DNA synthesis at 18 and 32°C within 1 h of culture, (ii) The G2 phase was observed to be less than 1 h. (iii) Both G1 and G2 microspores were induced to embryogenesis. (iv) Vegetative nuclei were always at G1 at the onset of culture and became labelled only at 32°C condition, sometimes within 4 h of culture. The data obtained will be published and discussed in more detail (see Binarova, P. et al. (1993) TAG (in press)).