Modulated 685-nm chlorophyll fluorescence was measured under steady-state conditions in pea leaves adapted to moderate actinic light. When the measurements were performed after a short exposure to heat (42 °C) in darkness, the maximal (F_m) and steady-state (F_s) fluorescence levels were dramatically and irreversibly quenched whereas the basic fluorescence F_o remained unchanged. Concomitantly , photo synthetic O₂ evolution was irreversibly inhibited. Analysis of the fluorescence data suggested that the heat treatment affected primarily the PS II reaction center, with the main effect being presumably an increased thermal dissipation of the excita­tion energy transferred to the reaction centers. In contrast, when heat stress was imposed in the presence of light, the loss of variable fluorescence (F_m-F_o ) was much less marked and was fully reversible. In addition , no inhibition of in vivo O₂ evolution was observed when the samples heated in the light were recooled at 25 °C. The results indicate that light acted as an efficient protector of PS II against heat injury.