Aim: To study the effect of intratesticular administration of ethane-1,2-dimethylsulphonate (EDS) which has been exten-sively used to selectively destroy Leydig cells in rats and study the role of gonadotropin in regulation of differentiation ofLeydig cells (LC) in the adult male bonnet monkey. Methods and Results: In vitro studies with cultured interstitialcells isolated from monkey testis revealed an inhibitory effect of EDS on LC as assessed by decrease in testosterone pro-duction. Intratesticular administration of EDS (5, 10, 20, 50 mg/testis) resulted in a dose-dependent rapid decrease inserum testosterone levels, with a 65 % decrease with 5 mg of EDS by the 3rd day, which returned to control levels by the45th day. EDS treatment resulted in a significant decrease in testicular testosterone. In addition a significant decrease in[(125)～I]hCG binding and phenylesterase activity in the interstitial cells was noticed. Histological analysis of the testes onthe 5th day after administration of EDS reveale Aim: To study the effect of intratesticular administration of ethane-1,2-dimethylsulphonate (EDS) which has been exten-sively used to selectively destroy Leydig cells in rats and study the role of gonadotropin in regulation of differentiation ofLeydig cells (LC) in the adult male bonnet monkey. Methods and Results: In vitro studies with cultured interstitial cells isolated from monkey testis revealed an inhibitory effect of EDS on LC as assessed by decrease in testosterone pro-duction. Intratesticular administration of EDS (5, 10, 20, 50 mg / testis) resulted in a dose-dependent rapid decrease in serum testosterone levels, with a 65% decrease with 5 mg of EDS by the 3rd day, which returned to control levels by the 45th day. EDS treatment resulted in a significant decrease in testicular testosterone . In addition a significant decrease in [(125) ~ I] hCG binding and phenylesterase activity in the interstitial cells was noticed. Histological analysis of the testes on the 5th day after administra tion of EDS reveale