Three independent components of hamster masculine behaviour (approaching, leaving and sniffing the female) have been shown to depend on both androgenic and oestrogenic action. The behavioural role of 5 alpha-reduced androgens was assessed by blocking 5 alpha-reduction of testosterone by means of 17 beta-N,N-diethylcarbamoyl-4-aza-5 alpha-androstan-3-one (4-MA) through slow-release silicone elastomer implants. Three dose levels of 4-MA were given to intact, sexually active males. The probability of approaching the female and the amount of sniffing directed to her were both decreased, while the probability of leaving the female was increased. Sniffing and the tendency to approach were affected at lower dose levels than was the tendency to leave. The higher dosage of 4-MA also abolished odour-based discrimination between females shown by normal males. These effects of 4-MA on behaviour were confirmed in castrated hamsters maintained on testosterone. The suppressive effects of 4-MA on behaviour were reversed by treatment with 5 alpha-dihydrotestosterone (DHT). We conclude that testosterone maintains sociosexual behaviour in part by conversion to its 5 alpha-reduced metabolites; components of this behaviour differ in their relative dependence on DHT. In this species, DHT is likely to be a behaviourally active metabolite of testosterone involved in the fine control of behaviour.