The masculinization of social play behavior in the rat is dependent upon the actions of androgens during the neonatal period. The amygdala, a major androgen-target region in the rat limbic brain, appears to be a critical site for this androgenic effect. We tested this hypothesis by implanting testosterone-bearing cannulae into the amygdala of female rat pups on Day 1 of life; the implants were removed on Day 8 of life. The animals were then observed daily between Days 26 and 40 of life and the frequency of play-fighting was recorded. Testosterone-implanted females, like normal males, engaged in significantly more play-fighting than did control females (implanted with cholesterol-bearing cannulae). We have also presented data indicating that the testosterone diffusion from the cannulae was, for the most part, restricted to the amygdala. Thus, testosterone implanted into the amygdala mimicked the effects previously reported for systemic testosterone injections, supporting the idea that the amygdala is a critical region for the actions of androgens on the sexual differentiation of social play behavior in the rat.