In vitro experiments are reported showing that psoralens and other furocoumarins of current pharmacological interest, e.g., angelicin and 4,6,4'-trimethylangelicin, all have, to a variable extent, the ability to sensitize the photooxidation of glutathione in ethanol/phosphate buffer with pyrex-filtered ultraviolet light. Besides substrate concentration and the nature of the furocoumarin used, the rate of the sensitized reaction is markedly dependent on the partial pressure of oxygen and the pH of the medium, being progressively faster on passing from pH 5 to pH 8.5. Scavengers of superoxide ions (superoxide dismutase), hydrogen peroxide (catalase) and singlet oxygen (sodium azide, diazabicyclooctane, sorbic acid) have little or no inhibitory effect on the reaction rate. These and other data suggest that furocoumarins can directly sensitize glutathione photooxidation by forming a charge transfer complex which is driven to the oxidized products in the presence of oxygen. The possible relevance of these results to the mechanisms of skin melanin hyperpigmentation induced by furocoumarins and ultraviolet light is discussed.