Plants containing aloin A, aloe emodin, and structurally related anthraquinones have long been used as traditional medicines and in the formulation of retail products such as laxatives, dietary supplements, and cosmetics. Since a recent study indicated that topically applied aloe emodin increases the sensitivity of skin to UV light, we examined the events following photoexcitation of aloin A and aloe emodin. We determined that incubation of human skin fibroblasts with 20 microM aloe emodin for 18 h followed by irradiation with UV or visible light resulted in significant photocytotoxicity. This photocytotoxicity was accompanied by oxidative damage in both cellular DNA and RNA. In contrast, no photocytotoxicity was observed following incubation with up to 500 microM aloin A and irradiation with UVA light. In an attempt to explain the different photobiological properties of aloin A and aloe emodin, laser flash photolysis experiments were performed. We determined that the triplet state of aloe emodin was readily formed following photoexcitation. However, no transient intermediates were formed following photoexcitation of aloin A. Therefore, generation of reactive oxygen species and oxidative damage after irradiation of aloin A is unlikely. Although aloin A was not directly photocytotoxic, we found that human skin fibroblasts can metabolize aloin A to aloe emodin.