Seven-day hypophysectomized rats were intracerebroventricularly (i.c.v.) injected with β-endorphin, ACTH1–10 or β-endorphin + ACTH1–10 (10–20 ng of each compound) and the [3H]flunitrazepam ([3H]FNZ) binding to the rat cerebral cortex of hypophysectomized rats was assayed one hour later. The i.c.v. injection of ACTH1–10 (10–20 ng) or β-endorphin (10–20 ng) significantly increased [3H]FNZ binding to a similar extent. The effect of i.c.v. injection of aCTH1–10 on brain binding was blunted by simultaneous β-endorphin administration at the same doses. The i.c.v. naloxone injection (10–20 ng) did not modify the effect of ACTH1–10 (10 ng) on [3H]FNZ binding, but counteracted, in a dose-related manner, the blocking effect of β-endorphin on ACTH1–10-dependent brain [3H]FNZ binding. The results suggest the existence of an opioid-melanopeptide integration to control brain benzodiazepine receptors.