The effects of LHRH and a potent LHRH agonist (LHRHa) on invitro testosterone production by enzyme-dispersed rat interstitial cells were evaluated. In a series of in vitro experiments, neither basal nor human menopausal gonadotropin (hMG)-stimulated testosterone production were significantly affected by doses of LHRH or LHRHa ranging from 10(-12)--10(-5) M. In addition, adult male rats were treated chronically with once daily injections of LHRH or LHRHa (2 micrograms/rat) or the vehicle for 1--7 days and decapitated 24 h after the last injection, and their testes were removed and weighed. Testicular weights decreased significantly by day 3 and were maximally decreased by day 6. In vitro testosterone production in response to 1--5 mIU human menopausal gonadotropin was markedly impaired (greater than 50%) in cells from rats treated with LHRHa for 2 days or longer and in rats treated with LHRH longer than 3 days. These data indicate that 1) LHRH and LHRHa do not alter in vitro testosterone production by dispersed rat interstitial cells and 2) interstitial cells of rats pretreated with LHRH and LHRHa exhibited impaired in vitro testosterone production. The data do not, however, rule out a direct effect of LHRH or LHRHa on testicular systems other than those involved in steroidogenesis.