As an alternative to testing the photobiological effects of various drugs with animals, in vitro photochemical reaction systems have been developed to act as a first screen to determine whether a compound has the capability to be involved in photobiological processes. This approach involves determining, by means of competitive kinetic reaction systems, the ability of the drug to photosensitise oxidation by a singlet oxygen or free radical mechanism. Free radical reactivity appears to be the factor which correlates more closely with in vivo photosensitivity. Also reviewed are studies on the photodegradation pathways of several of the more frequently reported drug photosensitisers, including hydrochlorothiazide, frusemide, naproxen, benoxaprofen, azathioprine and metronidazole.