The effects of drugs representing three classes of benzodiazepine (BDZ) receptor-acting agents on circulating corticosterone (CS), noradrenaline (NA) and adrenaline (A) were examined in unstressed rats. Intragastric administration of a single-dose of the inverse agonist 3-carbomethoxy-4-ethyl-6,7-dimethoxy-beta-carboline (DMCM; 10 mg/kg) evoked 15-, 4- and 1.5-fold increases in plasma CS, A and NA, respectively, as compared to control values. The DMCM-induced CS, A and NA rises were completely blocked by combined treatment with the BDZ antagonist flumazenil (Ro 15-1788; 20 mg/kg). Flumazenil given alone did not affect plasma hormone levels. Administration (either intragastrically or intraperitoneally) of a single-dose of the BDZ agonist chlordiazepoxide (CDP; 20 mg/kg) produced a 10- to 15-fold increase in plasma CS but caused no change in plasma NA and A contents. Pretreatment with flumazenil blocked the CDP-elicited release of CS. The findings indicate that the CNS mechanisms controlling pituitary-adrenocortical and sympatho-adrenal outflow under basal conditions are functionally linked to central-type BDZ receptor system(s). Drugs with agonist or inverse-agonist actions at these receptor sites can be differentiated from each other by their distinct effects on plasma NA and A, but not CS, release.