The effects of moderate light at chilling temperature on the photosynthesis of unhardened (acclimated to +18° C) and hardened (cold-acclimated) spinach (Spinacea oleracea L.) leaves were studied by means of fluorescence-induction measurements at 20° C and 77K and by determination of quantum yield of O2 evolution. Exposure to 550 μmol photons·m(-2)·s(-1) at +4° C induced a strong photoinhibition in the unhardened leaves within a few hours. Photoinhibition manifested by a decline in quantum yield was characterized by an increase in initial fluorescence (F o) and a decrease in variable fluorescence (F v) and in the ratio of variable to maximum fluorescence (F V/F M), both at 77K and 20° C. The decline in quantum yield was more closely related to the decrease in the F V/F M ratio measured at 20° C, as compared with F V/F M at 77K. Quenching of the variable fluorescence of photosystem II was accompanied by a decline in photosystem-I fluorescence at 77K, indicating increased thermal de-excitation of pigments as the main consequence of the light treatment. All these changes detected in fluorescence parameters as well as in the quantum yield of O2 evolution were fully reversible within 1-3 h at a higher temperature in low light. The fast recovery led us to the view that this photoinhibition represents a regulatory mechanism protecting the photosynthetic apparatus from the adverse effects of excess light by increasing thermal energy dissipation. Long-term cold acclimation probably enforces other protective mechanisms, as the hardened leaves were insensitive to the same light treatment that induced strong inhibition of photosynthesis in unhardened leaves.