Thylakoid membranes from spinach were irradiated with UV light of 254 nm. Photosynthetic electron flow driven by photosystem II, but not by photosystem I is fully inactivated in about 2-5 min. Inactivation of electron flow is prevented by inhibitors of the QB site of both the DCMU and of the phenol type. UV light inactivates electron How both under anaerobic and aerobic conditions. This is in contrast to white light where fast inactivation occurs only under anaerobic conditions and where only inhibitors of the DCMU-type protect. Inactivation of electron flow by UV light is followed on a slower time scale by a specific degradation of thylakoid membrane proteins. As shown by immunoblotting the D-1 protein and to a smaller extent also the D-2 protein subunit - that together form the reaction center of photosystem II - are de- graded during UV light irradiation. The disappearance of these proteins occurs only under aerobic conditions. Both types of QB site inhibitors prevent the degradation of the two plasto-quinone-binding proteins. A degradation product of the D-l protein is observed at about 8 kDa size. The results are discussed in their relevance to rapid turnover and photoinhibition in vivo and to the topology of the quinone-binding site in the D-1 and D-2 protein.