Photoinhibition of photosystem (PS) n was induced in primary leaves of 25 day-old peppers grown $100;{mu}mol{cdot}m^{-2}{cdot}s^{-1},;at;25^{circ}C$. The modulation of PSII functionality in vivo was induced by varying both irradiance ($0-3000;{mu}molm^{-2}{cdot}s^{-1}$) and duration (0-70 min) of light treatment. The functionality of PSII was investigated in terms of photochemical efficiency of PSII (Fv/Fm) and quantum yield of $O_2$ evolution, and expressed as a function of photon exposure [$mol;photons{cdot}m^{-2}$, the product of irradiance and duration of light treatment (Bell and Rose, 1981)]. Contrary to the linear decline of Fv/Fm ratio showing 50% decreases by absorption of $10;mol;photons{cdot}m^{-2$, quantum yield of $O_2$ evolution decreased biphasically with increasing photon exposure, showing 50% decreases by absorption of $5.5;mol;photons{cdot}m^{-2}$. Treatment of low temperature at $15^{circ}C$ for 30 min alone did not affect the functionality of PSII, but high temperature ($45^{circ}C$) significantly inactivated PSII activity. However, when Jeaves of pepper were subjected to low or high temperature in the presence of light, PSII was substantially photoinactivated. These results suggest the presence of different photoinhibitory mechanisms at low and high temperature.rature.