The role of endogenous opioids in the regulation of tonic and stress-induced prolactin secretion was studied in male rats. Animals with chronically indwelling intra-atrial catheters were used and served as their own controls. Intravenous injection of a potent rabbit anti-β-endorphin antiserum produced no change in either baseline serum prolactin or in the rise induced by swimming for 15 minutes at 20 C. Naloxone, 0.5 mg/kg intravenously, produced a small but statistically significant lowering of baseline serum prolactin levels, as well as a mild blunting of the stress-induced serum prolactin rise. The failure of anti- - endorphin antiserum to affect serum prolactin may be explained either by failure of the antiserum to gain access to hypothalamic prolactin regulating loci, or to lack of involvement of β-endorphin in the control of serum prolactin. The suppression of prolactin secretion seen with naloxone indicates that endogeneous opioids are involved in prolactin regulation, though the relatively small magnitude of the changes observed suggests that their role under physiological conditions may be a minor one.