Emerging evidences suggest that testosterone is an important regulator of body fat mass in men. However, the mechanism underlying the regulation of body composition by testosterone is not well understood. The paper demonstrates testosterone inhibited the activity of peroxisome proliferator-activated receptor gamma (PPARgamma), which controls the adipogenic differentiation and lipid metabolism. Pre-inhibition of androgen receptor did not influence the inhibition of PPARgamma activity by testosterone. Moreover, pre-treatment with testosterone attenuated the insulin-induced up-regulation of PPARgamma activity. These results provide a novel mechanism for the effect of testosterone on fat mass control.