After exposure of cells of the cyanobacterium Synechococcus PCC 7942 to increased irradiance, there is a good correlation between the rate of exchange of the ‘low-light’ form D1:1 for the ‘high-light’ form D1:2, and the rate of photosystem II photoinactivation (PSIIPI) assessed in chloramphenicol (CAP)-treated cells. This result indicates that PSIIPI triggers the exchange of D1:1 for D1:2. On the other hand, the reverse exchange of D1:2 for D1:1 after return of the cells to low irradiance is apparently independent of PSIIPI. The D1:2 degradation is partially inhibited by the protease inhibitor Pefabloc, suggesting the role of a serine protease in this process. The degradation of both D1:1 under high irradiance and D1:2 under low irradiance is slowed down in the presence of CAP, confirming previous results obtained with other cyanobacterial strains. A comparison of the turnover rates of the D1 forms under high irradiance shows that the radiolabelled D1:2 turns over significantly faster than D1:1. The presence of D1:2 in the membrane therefore improves the protection of PSII against photodamage and this can be the primary reason for the preferential synthesis of this form under high irradiance.