Three experiments using domestic chicks demonstrate that the extinction of a conditioned taste aversion to a green-coloured sucrose solution is slowed by the administration of testosterone or 5 alpha-dihydrotestosterone but is unaffected by estradiol. Testosterone slows extinction in male and female chicks to an equal extent. Previously described effects of testosterone on attentional and memory mechanisms have also been produced by low doses of estradiol so a separate mechanism must be postulated to explain the present results. It is probably similar to the mechanism responsible for a steroid-dependent slowing of extinction in rats since those effects are also androgenic and present in both sexes.