Testosterone is converted in the dove (Streptopelia risoria) brain to 5 beta-reduced metabolites that do not affect behavior. In long-term castrated birds, which are relatively insensitive to the behavioral effects of testosterone, the activity of preoptic 5 beta-reductase is increased. The increase, which is specific to the preoptic area, is reversed by estrogen. Inactivation of testosterone by 5 beta-reduction may be involved in the control of brain sensitivity to androgen.