The present study was conducted to examine roles of brain monoamines and opioid peptides in growth hormone (GH) secretion in unanesthetized, freely behaving rats. The administration of chlorpromazine (CPZ, 300 μg/100 g, i.v.), an antagonist of brain monoamines, to rats that were passively immunized with antiserum to somatostatin immediately lowered plasma GH levels and inhibited episodic GH secretion. An intraventricular injection of β-endorph¡n (3.5 μg) stimulated GH secretion. This effect was completely inhibited by the prior administration of naloxone (100 μg/100 g, i.v.), a specific antagonist of opioid peptides but not by CPZ. In addition, the administration of naloxone did not inhibit episodic GH secretion. The results suggest that CPZ inhibits episodic GH secretion via a factor(s) other than somatostatin. It is also inferred that brain monoamines. but not opioid peptides, play major roles in the regulation of episodic GH secretion in