The response of a number of species to high light levels was examined to determine whether chlorophyll fluorescence from photosystem (PS) II measured at ambient temperature could be used quantitatively to estimate the photon yield of O2 evolution. In many species, the ratio of the yield of the variable (FV) and the maximum chlorophyll fluorescence (FM) determined from leaves at ambient temperature matched that from leaves frozen to 77K when reductions in FV/FM and the photon yield resulted from exposure of leaves to high light levels under favorable temperatures and water status. Under conditions which were less favorable for photosynthesis, FV/FM at ambient temperature often matched the photon yield more closely than FV/FM measured at 77K. Exposure of leaves to high light levels in combination with water stress or chilling stress resulted in much greater reductions in the photon yield than in FV/FM (at both ambient temperature and 77K) measured in darkness, which would be expected if the site of inhibition was beyond PSII. Following chilling stress, FV/FM determined during measurement of the photon yield in the light was depressed to a degree more similar to that of the depression of photon yield, presumably as a result of regulation of PSII in response to greatly reduced electron flow.