Abstract Adrenal corticoid release is a major component of the stress response which can affect many body functions including behavior. The purpose of our studies was to examine the effects of corticosterone (B) on both the agonistic and courtship components of social behavior in male side-blotched lizards ( Uta stansburiana). Of particular interest was the effect of B on plasma testosterone (T), a hormone known to influence aggression, and the importance of this action on the behavioral effects of B. Experiments included either castrated males, hormone (either B, T, or B + T)-implanted intact males, or sham males. Behavioral observations were recorded when these males were challenged with "immigrant" males; measurement of plasma steroids confirmed the efficacy of implants in elevating B and T. Castration, B implantation, and combined B and T implantation significantly reduced aggressive behavior to varying degrees, while T implants in intact animals had no effect. B implants significantly decreased plasma T levels (from 1.87 to 0.33 ng/ml), but this decrease was not essential for the inhibitory effect of B on aggression since B + T implantation also reduced aggression, even though plasma T was elevated above normal (67.6 ng/ml). In contrast, B implantation did not affect male courtship and copulatory behavior when males were presented with estrogenized females. These results suggest that the effect of B on social behavior is not through just a single route, that of decreasing plasma T, and that B can affect various intra- and intersexual behaviors differently.