Previous studies carried out in normal male or ovariectomized female rats have shown that bombesin plays an inhibitory role on growth hormone (GH) secretion. Since estrogens play an important role in the neuroregulation of GH secretion, we have studied the effects of bombesin on basal GH secretion and GH responses to GH-releasing hormone (GHRH) in untreated and estrogen-treated male rats (200 micrograms estradiol valerate s.c., 1 single dose 3 days before the experiment or every 3 days for 2 weeks). All the experiments were carried out in rats anesthetized with pentobarbital. GH responses to GHRH (1 microgram/kg) were inhibited by bombesin (100 micrograms/kg) in untreated rats, but were markedly increased in rats treated with estrogens either 3 days before or for the previous 2 weeks. Similarly, bombesin administration (25 or 100 micrograms/kg) in estrogen-treated rats induced a clear-cut, dose-related increase in basal GH levels. This stimulatory effect of bombesin was not affected by passive immunization with antisomatostatin antiserum (750 microliters i.v., 60 min before) and only partially blocked by anti-rGHRH antiserum (750 microliters i.v., 1 h before). In conclusion, our data show that bombesin exerts an inhibitory effect in normal male rats but a stimulatory one in estrogenized rats. This latter effect is independent of somatostatin and only partially blocked by anti-rGHRH serum.