High-light treatments (1750-2000 μmol photons m(-2) · s(-1)) of leaves from a number of higher-plant species invariably resulted in quenching of the maximum 77K chlorophyll fluorescence at both 692 and 734 nm (F M, 692 and F M, 734). The response of instantaneous fluorescence at 692 nm (F O, 692) was complex. In leaves of some species F O, 692 increased dramatically in others it was quenched, and in others yet it showed no marked, consistent change. Regardless of the response of F O, 692 an apparently linear relationship was obtained between the ratio of variable to maximum fluorescence (F V/F M, 692) and the photon yield of O2 evolution, indicating that photoinhibition affects these two variables to approximately the same extent. Treatment of leaves in a CO2-free gas stream containing 2% O2 and 98% N2 under weak light (100 μmol · m(-2) · s(-1)) resulted in a general and fully reversible quenching of 77K fluorescence at 692 and 734 nm. In this case both F O, 692 and F M, 692 were invariably quenched, indicating that the quenching was caused by an increased non-radiative energy dissipation in the pigment bed. We propose that high-light treatments can have at least two different, concurrent effects on 77K fluorescence in leaves. One results from damage to the photosystem II (PSII) reaction-center complex and leads to a rise in F O, 692; the other results from an increased non-radiative energy dissipation and leads to quenching of both F O, 692 and F M, 692 This general quenching had a much longer relaxation time than reported for ΔpH-dependent quenching in algae and chloroplasts. Sun leaves, whose F V/F M, 692 ratios were little affected by high-light exposure in normal air, suffered pronounced photoinhibition when the exposure was made under conditions that prevent photosynthetic gas exchange (2% O2, 0% CO2). However, they were still less susceptible than shade leaves, indicating that the higher capacity for energy dissipation via photosynthesis is not the only cause of their lower susceptibility. The rate constant for recovery from photoinhibition was much higher in mature sun leaves than in mature shade leaves, indicating that differences in the capacity for continuous repair may in part account for the difference in their susceptibility to photoinhibition.