Adult sheep which has been castrated either before or after puberty were treated with a variety of steroids. The administration of testosterone propionate, oestrone, oestradiol-17 beta or diethylstilboestrol to animals castrated before puberty caused them to mount oestrous ewes. Oestradiol-17 alpha was less effective than these hormones in this regard, whilst oestriol, hexoestrol and 5 alpha-dihydrotestosterone were ineffective. The response to oestradiol-17 beta was not altered by the concurrent administration of dexamethasone to block the pituitary-adrenal axis which suggests that oestradiol-17 beta was not exerting its effect indirectly by causing the release of adrenal steroids. When 5 alpha-dihydrotestosterone was administered in conjunction with oestradiol-17 beta intromission and ejaculation were observed in addition to mounting behaviour. When rams were castrated as adults their mating behaviour slowly declined over the course of 2 years. After this time, mounting behaviour was rapidly restored by the administration of oestradiol-17 beta but not by 5 alpha-dihydrotestosterone. These results are consistent with the hypothesis that oestrogens are the ultimate agents responsible for promoting mating behaviour in male animals and hence aromatizable androgens, such as testosterone, are effective whereas non-aromatizable androgens, such as 5 alpha-dihydrotestosterone, are not.