Exposure of intact rice leaves to an irradiance of 1000 mumol m(-2) s(-1) at 6 degrees C for 2 h caused severe photoinhibition of Photosystem II. The rate and extent of photoinhbition were greatly exacerbated in leaves fed with 10 mM reduced glutathione (GSH) or 10 mM cysteine. Analyses of antioxidant enzyme activities as well as the application of protein synthesis inhibitors revealed that the increased sensitivity to photoinhibition following GSH feeding was not related to its effect on cellular antioxidant systems. On the other hand, feeding with GSH markedly suppressed the formation of zeaxanthin and antheraxanthin via the xanthophyll cycle and its associated nonradiative energy dissipation in leaves chilled in high light, suggesting that the stimulating effect of exogenous GSH on photoinhibition may be attributable to its action on the xanthophyll cycle. In vitro experiments using isolated thylakoids indicated that GSH is a weak inhibitor of violaxanthin deepoxidation. The possible implications of these results are discussed.