The relative values of the rate constants of photochemical and nonphotochemical processes of energy dissipation in photosystem (PS) II were estimated from the initial (F₀) and maximal (F_m) levels of modulated chlorophyll fluorescence measured in intact (Pisum sativum, Solanum tuberosum) leaves adapted to an actinic light of moderate intensity (15 to 30 W m^-2). Pretreatment of the leaves with a strong light (300-400 W m^-2) caused pronounced changes in F₀ anf F_m. reflecting a marked decrease in the rate constant k_{2 b} of PS II photochemistry and a concomitant increase in the combined rate constant (k_N-k₂₂) of all nonphotochemical processes of PS II deactivation (excluding unit-unit energy transfer, k₂₂). Although the above changes were accompanied by a substantial lowering of the photochemical efficiency of PS II, the so-called coefficient for photochemical fluorescence quenching q_P remained largely unchanged. The light-induced modifications of the k_{N -}K_{22 and} k_{2 b} values were observed after pretreatment at light intensities higher than around 100 W m^-2, had a relatively slow time response (in the order of 30min), were reversible, temperature-dependent and intensitive to dithiothreitol. This dynamic photoregulation of the rate constants of the photochemical and nonphotochemical pathways of pigment deexcitation in PS II could represent a physiological mechanism preventing overexcitation and subsequent photoinhibitory damage of PS II reaction centers.