Induction of DNA damage as a consequence of exposure to UV light has been established as the major and still increasing cause of skin cancer. Absorption of the photon energy may be either directly by the DNA molecules (for wavelengths < 320 nm) or may be by endogenous or exogenous chemicals (sensitizers) with the potential of energy or electron transfer to DNA. Oxygen-mediated reactions (often called type II reactions) appear to be the most important mechanism since molecular oxygen is a good and abundant substrate for triplet excited sensitizers. Energy transfer to molecular oxygen is possible for wavelengths in the near UV and in the visible part of the solar spectrum since the energy of the excited oxygen molecule ((1)O2*) is comparatively low. A few light-absorbing pharmaceuticals have long been known to cause photo(geno)toxic effects. Notably psoralene and chlorpromazine derivatives have been established as photomutagens and the reaction mechanisms have been identified. The fluoroquinolone antibiotics have more recently been recognized as being photomutagenic. The type of DNA damage and the modulation by antioxidants indicate the involvement of reactive oxygen species (ROS) but other mechanisms are also reported at least for some derivatives. In routine genotoxicity studies we observed a photomutagenic activity of a compound under development as an anxiolytic agent in the Ames tester strain TA102 at 'normal laboratory illumination' conditions. Further investigations showed strong photogenotoxic activity in tests for gene mutations and chromosomal aberrations in mammalian cells. The compound proved to be a potent (1)O2-producer. The finding led to termination of development but in the course of studies several structural analogues have been tested for which structure activity relationships will be described. The relevance of photogenotoxic properties of drugs for predicting adverse effects in man will be discussed.