The kinetic properties of the testosterone-metabolizing enzymes were studied in the hypothalamus of adult and young zebra finches of both sexes. Estradiol, 5 alpha-dihydrotestosterone, 5 beta-dihydrotestosterone and 5 beta-androstane-3 alpha,17 beta-diol were identified as metabolites of testosterone in males and females at different ages between 5 days post-hatch and adulthood. During maturation, the maximum velocity (Vmax) of the aromatase and 5 beta-reductase decreased in males and females while the affinity of these enzymes for the substrate increased (decrease in km). These changes were more pronounced for 5 beta-reductase than for aromatase. The affinity change of the 5 beta-reductase occurred progressively during the post-hatching development between 5 and 30 days and is thus probably a true developmental process. In the case of the aromatase, the change in affinity occurred much later (after 30-40 days) and is thus probably related to the sexual maturation. These kinetic changes during development are directly related to the roles played by testosterone and its metabolites, in particular estradiol, in the differentiation and activation of reproductive behavior in the zebra finch. In particular, the dramatic decrease in 5 beta-reductase activity during sexual maturation should correspond to a potentiation of testosterone action in the brain.