High (HI, 200 W m'^) and low (LI, 30 W m"^) irradiance treatments on the cells of cyanobactenum Synechococcus elongatus Nfig., var. thermalis Geitl. strain KOVROV 1972/8 were perfonned in the presence of chloramphenicol (CAP), and with addition of DCMU and hydroxylamine (HA), respectively, at growth (56 °C) and low (20 °C) temperatures, to distínguish allegedly different mechanisms of PS 2 photoinactivation (PS 2 PI). At both temperatures HI caused a decline of Fy and Hill reaction activity (HRA) followed by degradation of Dl and to a lesser extent also D2 protein. Fq increased slowly during irradiance at 56 while at 20 ®C it quickly rose to constant level. Degradation of proteins was slowed at a lower temperature. The presence of DCMU during photoinhibition significantly blocked the Fq rise and also prevented PS 2 protein degradation at both temperatures. The course of PS 2 PI under LI resembled tlmt, observed at HI, but changes were much slower. During irradiation of the cells, in which oxygen evolving complex (OEC) was impaired by HA, we observed: (/) at least a ten-fold faster decline of PS 2 electron transport activity than in the cells vňth fimctional OEC under the same conditions; (2) an extensive degradation not only of Dl and D2, but also of the apoprotein of chlorophyll-protein CP43 {ACP43y, (3) almost complete inhibition of PS 2 protein degradation in the presence of DCMU. Thus under all conditions tested in vivo which do not affect the fimction of OEC, the fimction of OEC, PS 2 PI proceeds via the acceptor side and a fimctional impairment of OEC is necessary for induction of the donor side mechanism. When OEC is impaired (e.g. by HA) this mechanism can come in action.