Oxygen evolving Photosystem II particles were exposed for up to 10 h to 100 W m(-2) white light at 20°C under aerobic, low oxygen, strictly anaerobic and strongly reducing conditions. The fast and slow photoinactivation processes described earlier (Šetlík et al. 1989) were observed during the first 120 min. The third and by far the slowest process impaired the primary charge separation P680(+)-Pheo(-). Its half-time was about 2.5 h under aerobic and strongly reducing conditions and about 4 h under anaerobic and low oxygen conditions. In these time intervals there were no changes in the chlorophyll-protein and polypeptide composition of the particles irradiated under anaerobic, low oxygen or strongly reducing conditions while a dramatic degradation of chlorophyll-proteins and polypeptides occurred under aerobic conditions.