Oestrogens administered to adult males are known to suppress the synthesis of androgens in the testicles. A main effect of this treatment on testicular steroid metabolismin vitroseems to be a significant reduction of 17α‐hydroxylase activity. A concomitant increase in the 20α‐hydroxysteroid dehydrogenase activity was, however, observed in four young oestrogen‐treated transsexual males (Rodriques et al. 1976).We have analysed the metabolism of [3H]progesteronein vitroin testicular tissue incubates derived from six males on current oestrogen treatment and from one male 19 weeks after cessation of oestrogen therapy.The metabolism in general of [3H]progesteronein vitrowas observed to be decreased in all males on oestrogen treatment as compared to control subjects. The metabolite 20α‐dihydroprogesterone was found to constitute between 85–91 per cent of all metabolites formed in the patients under treatment.On the contrary, the metabolic pattern observed in the previously treated male, appeared to be fully consistent with the patterns observed in non‐treated control subjects. Thus, the main alterations in intratesticular progesterone‐metabolism caused by exogenous administration of oestrogens, at least for 23 consecutive weeks, seem to be compl