In order to determine the importance of reduced and aromatized metabolites of testosterone for male sexual behavior in Peromyscus maniculatus bairdi, castrated males were treated with 5 alpha-reductase and aromatase inhibitors. In the first experiment, testosterone propionate (TP) activation of male copulatory behavior was blocked by the administration of the 5 alpha-reductase inhibitor 4-androsten-3-one-17-beta carboxylic acid (17 beta C). These treatments also prevented TP stimulation of seminal vesicles and ventral prostate gland weight. The inhibitory effects of 17 beta C were specific to testosterone, since 17 beta C did not prevent dihydrotestosterone propionate (DHTP) induction of male sexual behavior or seminal vesicles and ventral prostate gland weight increases. In the second experiment, TP activation of male copulatory behavior was prevented by the administration of the aromatase inhibitor 1,4,6-androstatriene-3,17-dione (ATD). The ATD did not interfere with DHTP activation of male reproductive behavior. Also, TP and DHTP stimulation of accessory sex organ weight was not blocked by ATD. On the basis of these data, it is suggested that metabolism of testosterone to both 5 alpha-reduced androgens and estrogens is obligatory for testosterone to reliably stimulate male sexual behavior in castrated male deer mice.