It has been suggested that testosterone is less effective at inducing crowing behaviour in young birds than in adults because of the presence of higher levels of steroid 5 beta-reductase in the young brain, which converts testosterone to inactive 5 beta-reduced metabolites. This hypothesis was tested indirectly by comparing the relative potencies of 5 alpha-dihydrotestosterone (5 alpha-DHT), which cannot be converted to 5 beta-metabolites, and testosterone at inducing crowing in young gonadectomized male and female quail. The promotion of cloacal gland growth by these treatments was also assessed since there are no age-related changes in 5 beta-reductase in this organ. Silicone elastomer implants (2 X 5, 5 and 10 mm) containing 5 alpha-DHT were more effective at stimulating crowing than similar implants of testosterone whilst there was little difference in their potency at inducing cloacal gland growth. These results are consistent with the hypothesis that brain steroid 5 beta-reductase regulates the behavioural activity of testosterone in the brain of young birds.