Adult mice, rats and hamsters were injected with 0 or 0.3 IU hCG/g BW, 24 h before sacrifice. Basal LH receptor concentration was highest in rats and lowest in hamsters (rats greater than mice greater than hamsters). Injection of hCG caused LH receptor down-regulation in rats and mice, and up-regulation in hamsters. Basal plasma progesterone was highest in hamsters and lowest in rats (hamsters greater than mice greater than rats), however, hCG increased plasma progesterone levels in mice and rats, but not in hamsters. Mice had much higher plasma and testicular testosterone levels than other species, but hCG did not induce a relatively more dramatic increase in any species. When testes fragments were incubated with 0 or 12.5 mIU hCG/ml for 4 h, hCG increased media progesterone levels in rats and control mice, but not in hamsters and hCG-injected mice. Also, hCG elevated media testosterone levels in control but not in hCG-injected animals. Furthermore, addition of hCG in vitro partially prevented the elevation of media testosterone induced by in vivo hCG. The present results indicate that the mechanisms for the transduction of the gonadotropic signal by the Leydig cells are species-defined.