The metabolic fates and modes of excretion of diethylstilboestrol mono [35S] Diethylstilboestrol has been used as a synthetic oestrogen in humans and in food-producing animals. It has been shown to have carcinogenic properties in a number of animal species (Umberger, 1975) and its use has been associated with the occurrence of tumours in humans (Herbst et al., 1972).The metabolic fate of diethylstilboestrol has been studied in several animal species (Abou-EI-Makarem et al., 1967) and it has been shown that biliary excretion as a glucuronic acid conjugate represents a major pathway for elimination (Klaasen, 1973). The disulphate ester of diethylstilboestrol, which is approximately half as active as diethylstilboestrol itself with respect to oestrogenic activity (Bishop et al., 1951), has been used as a water-soluble form of the synthetic oestrogen. Nevertheless, the metabolism of sulphated forms of diethylstilboestrol has not been reported.