The involvement of the endogenous benzodiazepine, octadecaneuropeptide (ODN), in the regulation of proopiomelanocortin (POMC) mRNA expression at the pituitary level, and the influence of adrenal and gonadal steroids, have been studied using a quantitative in situ hybridization technique. I.c.v. injection of ODN (4 micrograms/kg) in sham-operated rats induced a 17 and 7% decrease in the POMC mRNA expression in anterior and intermediate pituitary lobes respectively. To determine the reciprocal involvement of adrenal and gonadal steroids in this regulation, animals were adrenalectomized and/or castrated. Adrenalectomy significantly increased POMC mRNA expression by 48% at the anterior pituitary level, but induced a 10% decrease of hybridization signal at the intermediate pituitary lobe (vs control sham-operated). Adrenal ablation reversed the effect induced by ODN and increased POMC mRNA expression at the anterior and intermediate pituitary levels by 60 and 10% respectively, compared with control sham-operated. By contrast, castration, which produced a decrease in POMC mRNA in the anterior pituitary and an increase in the intermediate lobe, did not modify the negative influence of ODN observed in sham-operated animals. When rats were adrenalectomized and castrated, the adrenalectomy influence was predominant at the anterior pituitary level, since ODN increased significantly the hybridization signal (+68% vs control sham-operated), while the castration influence was predominant at the intermediate pituitary level, since ODN induced an 11% decrease in POMC mRNA signal compared with control sham-operated. These studies indicate that, in vivo, the decrease in POMC mRNA expression in the anterior and intermediate pituitary induced by an endogenous benzodiazepine is differently modulated by adrenal and gonadal steroids, with a predominant influence of adrenal steroids at the anterior pituitary level and gonadal steroids at the intermediate pituitary level.