Bombesin and other related peptides isolated from the skin of anuran species and found in mammalian brain stimulate PRL and GH release in steroid-primed male rats. On a molar basis, bombesin and alytesin are the most active peptides in this assay system. Their PRL- and GH-releasing activity is not affected by the type of anesthetics used, but their minimal effective dose is lower after intravenous administration as compared with intracisternal administration. The in vivo stimulatory effect of bombesin on PRL secretion is not modified by the histamine antagonist diphenhydramine or the opiate antagonist naloxone. By contrast, naloxone reverses the GH-releasing activity of bombesin, suggesting an opiate-dependent mechanism of action of this peptide on GH secretion. The absence of an in vitro effect of bombesin and related peptides on PRL and GH secretion and a minimal effective dose of approximately 30 ng in bombesin and alytesin administered in steroid-primed rats, make them the most active peptides reported so far to act on the brain to modify PRL and GH secretion.