SUMMARY Adult male rats were treated with daily s.c. doses of 10 mg. cyproterone acetate for 3 weeks. Histological examination of the exorbital lacrimal glands at the end of treatment showed distinct differences compared with male controls: the histological picture showing greater resemblance to the glands of female controls. In orchidectomized rats receiving substitution treatment with testosterone propionate, cyproterone acetate 'neutralized' the effect of testosterone propionate on the exorbital lacrimal gland to a considerable extent. The possibility that this gland may play a part in the sexual behaviour of the rat is discussed.