Departments of Biochemistry (Division of Chemical Endocrinology) and Endocrinology, Growth and Reproduction, Erasmus University, Rotterdam, The Netherlands (Received 22 October 1974) Administration of oestrogenic hormones to intact male rats has been reported to cause a decrease in testicular testosterone (T) production without affecting peripheral levels of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) (Danutra, Harper, Boyns, Cole, Brownsey & Griffiths, 1973; Chowdhury, Tcholakian & Steinberger, 1974), thus suggesting a direct effect of oestrogens on testicular processes. In contrast, Verjans, de Jong, Cooke, van der Molen & Eik-Nes (1974), after injections of oestradiol benzoate, observed a decrease in blood and testicular T concomitant with a decrease of gonadotrophins in the circulation. In order to elucidate the reason for these conflicting results, in the present study, oestradiol (E2) was administered as described by Danutra et al. (1973), while gonadotrophins were measured by the method used by Verjans et al. (1974). In addition