Binding kinetics of some photosystem 2 (PS 2)-directed ínhibitors to isolated pea thylakoids during photoinUbition were studied. Changes in the herbicide binding activity were overshadowed by the redox statě of the plastoquinone (PQ) pool. After extraction of the PQ pool from the thylakoids, a correlation between the decline of herbicide binding activity and the electron transfer activity during strong irradiation was found in the presence of oxygen; in anaerobiosis this correlation was absent. The opposite was observed for the binding kinetics of the electron transfer acceptor 2,6-dichlorophenol indophenol, that was affected faster under anaerobic conditions. Following irradiation, a movement of damaged Dj-protein products from the appressed to the non-appressed regions of the membrane was observed. This observation was correlated to the fínding of an increased level of atrazine bound to the non-appressed regions of irradiated membranes.