This study examines the role of dynorphin-A(1-13) and dynorphin-A(1-10)-amide in the neuroendocrine regulation of anterior pituitary hormones in nonrestrained, adult male rhesus monkeys. The effects of these opioids on plasma concentrations of prolactin (PRL), luteinizing hormone (LH), follicle-stimulating hormone (FSH), thyrotropin (TSH) and growth hormone (GH) were assessed. Intravenous administration of dynorphin-A(1–13), 1–120 µg/kg, significantly increased plasma PRL levels. Average maximal increases of 90–230% occurred within 5 min and levels remained significantly elevated for up to 120 min. PRL response reached a plateau following the 30 µg/kg dose. Dynorphin-A(1–13) had no observable effects on plasma concentrations of LH, FSH, TSH or GH at any dose level studied. Administration of dynorphin-A(1–10)-amide produced significant dose-dependent increases in plasma PRL concentrations. Dose levels of 1–120 µg/kg produced mean peak increases from 100 to 230%, 5–10 min postadministration. Dynorphin-A(1–10)-amide had no significant effect on plasma concentrations of LH, FSH, TSH or GH. The increases in plasma PRL concentrations induced by dynorphin-A were naloxone-reversible. These results indicate a selective effect of dynorphin-A on the regulatory mechanisms of PRL secretion over that of other anterior