Photoinhibition was induced in a cyanobacterium strain, Synechocystis 6714, and a green alga, Chlamydomonas reinhardtii, by exposing them to light intensities from 1000 to 4000 microE/(m2.s) at various temperatures. The photoinhibition process was followed by measurements of chlorophyll fluorescence and oxygen evolution. During exposure to high light, fluorescent active reaction centers II became low fluorescent inactive centers. This process involved several reversible and irreversible steps. The pathway from the active state to the inactive low fluorescent state was different in Synechocystis and Chlamydomonas. In the latter there was a reversible intermediary step characterized by an increase of F0. This state was stable at 5 degrees C and slowly reversible at room temperature. The high F0 fluorescence level corresponded to a state of photosystem II centers that were inactive for oxygen evolution. An F0 decrease occurred in the dark in the absence of protein synthesis and was correlated to a restoration of oxygen evolution. Further experiments suggested that the existence of the intermediate fluorescent state is due to modified closed centers in which the reduced primary acceptor is less accessible to reoxidation. In cyanobacteria this reversible state was not detected. In both organisms, the decrease of Fmax reflected an irreversible damage of photosystem II centers. These centers need replacement of proteins in order to be active again. The quenching of Fmax and the irreversible inhibition of oxygen evolution were slowed down in both organisms by decreasing the temperature of the photoinhibitory treatment from 34 to 5 degrees C. We conclude that low temperature protected the reaction center II from irreversible photodamage.