Picrotoxin (1 mg/kg, i.p.) and a high dose of diazepam (10 mg/kg, i.p.) increased the concentration of plasma corticosterone in nonstressed rats. This effect of diazepam was unaffected by picrotoxin and bicuculline (GABA-A receptor blockers), atropine (a muscarinic receptor blocker), apomorphine (a dopamine receptor agonist), haloperidol (a dopamine receptor blocker) and yohimbine (an alpha-2-adrenergic receptor blocker); but was blocked by clonidine (an alpha-2-receptor agonist) and this effect of clonidine was reversed by yohimbine. Diazepam was unable to elevate plasma corticosterone levels in rats pretreated with dexamethasone. Clonidine and the GABA-B receptor agonist, baclofen, failed to affect the picrotoxin-induced rise of plasma corticosterone, but this rise was abolished by a low dose of diazepam (1 mg/kg). The results suggest that the diazepam-induced enhancement of ACTH release, presumably mediated by blockade of alpha-2-adrenergic receptors, is responsible for the observed increase of plasma corticosterone level in rats. On the other hand, the elevation of plasma corticosterone induced by picrotoxin appears to be mediated by GABA-A receptors.