The ability of two proopiomelanocortin-derived peptides, alpha MSH and corticotropin-like intermediate lobe peptide (CLIP) [ACTH (18-39)] to antagonize the stimulation of PRL secretion by beta-endorphin (beta EP) was studied in the rat. When 50 ng beta EP were injected into the lateral cerebral ventricle, plasma PRL rose from a mean baseline of 1.87 +/- 0.43 ng/ml (+/- SEM) to a peak of 23.0 +/- 3.67 ng/ml 10 min after the injection. When the same animals received 500 ng alpha MSH together with 50 ng beta EP, the peak concentration of PRL was reduced by 74% to 6.05 +/- 1.43 ng/ml (P less than 0.005). After the injection of 500 ng CLIP together with 500 ng beta EP, the peak concentration of PRL was reduced by 47% to 12.8 +/- 3.09 ng/ml (P less than 0.01). Total PRL release, determined by calculating the areas under the plasma PRL concentration curves, was also significantly reduced by the injection of alpha MSH or CLIP. A dose of 100 ng alpha-MSH or CLIP also antagonized the stimulation of PRL secretion by 50 ng beta EP. PRL release was reduced by 62% after administration of 100 ng alpha MSH (P less than 0.001) and by 43% after 100 ng CLIP (P less than 0.05). When 100 ng alpha MSH and 100 ng CLIP were injected together, there was an additive effect in blocking the stimulation of PRL release by beta EP, and the peak plasma PRL concentration was reduced by 81%. Des-acetyl alpha MSH, the predominant form of alpha MSH in the hypothalamus, was also very effective in antagonizing beta EP-induced PRL release. The peak PRL concentration was reduced by 52% after administration of 100 ng des-acetyl alpha MSH plus 50 ng beta EP compared with that after beta EP alone (P less than 0.005). We conclude that relatively low doses of both alpha MSH and CLIP can effectively antagonize the actions of beta EP on pituitary PRL release. These findings suggest the possibility that differential posttranslational processing of proopiomelanocortin may serve as a regulator of anterior pituitary function.