This study is to investigate the effects of testosterone on adipogenesis and its molecular mechanism using RT-PCR analysis and transient transfection assays. Castrated(CAST) mice treated with testosterone had lower white adipose tissue weights and expression of adipocyte-specific genes($PPAR{gamma}$ and aP2) than CAST control mice. Consistent with the in vivo data, testosterone treatment inhibited triglyceride accumulation and expression of adipocyte-specific genes($PPAR{gamma}$ and aP2) in differentiated 3T3-L1 cells compared with control group. Testosterone-activated androgen receptor(AR) repressed the luciferase reporter gene activity induced by $PPAR{gamma}$ transfection. Thus, these results suggest that testosterone downregulates the actions of $PPAR{gamma}$ on adipogenesis through AR.