The antigonadal effects of [Ac-D-NAL(2)1, 4FD-Phe2, D-TRP3, D-Arg6]-LHRH (LHRH-A), a potent antagonist of LHRH, were investigated in rats and rabbits. Rats and rabbits were given LHRH-A (1250 micrograms/kg) daily for 15 days. Some animals were killed on day 16 (24 h after the last treatment) while others were mated. In the male rats serum LH and testosterone levels as well as the weights of sexual organs were significantly reduced. Mating behavior and fertility that were suppressed by the end of treatment returned to normal by 7 weeks after last treatment. In contrast to rats, the testicular function and fertility of rabbits appeared unaffected by LHRH-A treatment. The difference in the response between rats and rabbits led us to compare the response of rats and mice. Male rats and mice were given LHRH-A (1450 micrograms/kg) daily for 5 days and killed on day 6. In rats LHRH-A caused a 93% decrease in serum T and 88% decrease in in vitro testicular T production. In mice, however, the Leydig cell function remained unaffected when examined 24 h after the last dose of LHRH-A. To explain the differences between the effects of LHRH-A on rats, rabbits and mice, the acute effect of this peptide on serum T levels was investigated in these species. Administration of a single dose of LHRH-A (1250 micrograms/kg) led to a rapid decrease in serum T that was sustained for 24 h in rats. In rabbits and mice, however, the same dose of LHRH-A caused only a transient decrease in serum T. Male rhesus monkeys treated with LHRH-A (1000 micrograms/kg) also showed a transient decrease in serum T concentrations. It is concluded that there are considerable species differences in the sensitivity to the antigonadal effects of LHRH-A.