The effects of intraventricular (i.v.t.) morphine sulfate (MS) and β-endorphin (β-EP) on pituitary-adrenal activity and the release of pituitary β-EP were studied in rats. Pituitary-adrenal activity was monitored by measuring plasma corticosterone (CS) levels. 45 min after i.v.t. injection, both MS and β-EP caused dose-related increases in plasma CS, with β-EP being approximately ten times more potent on a molar basis. MS injected i.v.t. at 0.3, 1.0, 3.0 and 10.0 μg did not cause a significant reduction in pituitary immunoreactive (i.r.) β-EP, but did cause an increase in plasma i.r. β-EP at 3 μg of MS. β-EP injected i.v.t. at 1.5 μg caused a reduction of pituitary i.r. β-EP. Since i.v.t.-injected β-EP may have contributed to the measured plasma i.r. β-EP, a nonimmunoreactive analog (Des-Asn<sup>20</sup>-β<sub>c</sub>-EP) was used to assess the change in plasma i.r. β-EP. 5 μg of Des-Asn<sup>20</sup>-β<sub>c</sub>-EP injected i.v.t. caused increases in plasma i.r. β-EP and CS, as well as a 40% reduction in pituitary i.r. β-EP. The concomitant intraperitoneal (i.p.) injection of naloxone HC1 (10 mg/kg) significantly blocked the increase in plasma CS induced by 5 μg of β-EP. When naloxone HC1, 10 mg/kg was injected alone, a significant increase in plasma CS was found. The results indicate that i.v.t. β-EP is more potent than MS in causing the release of pituitary ACTH and β-EP. These findings are consistent with a role for brain endorphins in the regulation of CRF release.