The purpose of this study was to identify agents capable of regulating the release of biologically active ACTH from the isolated neuro-intermediate lobe of the rat pituitary. Agents found to be potent secretagogues included acetylcholine (100 mug/ml), hypothalamic stalk-median eminence extract (0.33 eq), arginine antidiuretic hormone (100 mU/ml) and serotonin (100 mug/ml). Lower doses of arginine antidiuretic hormone (5.5 mU/ml) and serotonin (2 mug/ml) were ineffective. Dopamine 2 and 5 mug/ml) inhibited the release of biologically active ACTH whereas norepinephrine (5 mug/ml) did not. Dexamethasone (0.25 mug/ml) did not alter the basal or stimulated release of ACTH from the isolated neuro-intermediate lobe in contrast to its effect on ACTH release from the isolated anterior pituitary. Similarly, the tripeptide, prolyl-leucyl-glycinamide, which has been reported by some to inhibit MSH release, had no effect on either the basal or stimulated release of ACTH. The data suggest that regulation of ACTH release from the neuro-intermediate lobe in vivo may involve both stimulatory (acetylcholine) and inhibitory (dopamine) inputs.