The influence of aromatization of testosterone occurring in adipose tissue has been studied in castrated obese male rats. The treatment for 30 days with a subliminal dose (115 micrograms/kg per day) of testosterone propionate improved significantly the sexual performance of castrated obese male rats as compared to the nonobese controls. Untreated castrated obese animals did not show any enhancement of sexual behavior. Furthermore, obesity failed to improve sexual performance in intact male rats. Measurements of plasma levels of testosterone and 17 beta-estradiol showed an increase in aromatization of testosterone in obese rats. Present data suggest that adipose tissue can be considered an important site for aromatization of testosterone to 17 beta-estradiol, and that aromatization is involved in the activation of sexual behavior in castrated male rats.