In anesthetized rats, concentrations of adrenaline and noradrenaline in sequential 20-min microdialysis samples were measured. Central activation of adrenal secretory function induced by controlled bleeding, increased catecholamine levels in dialysate but the maximal levels were reached after hemorrhage when a decrease in the blood pressure had been stabilized. The results of parallel measurements of catecholamine concentration in dialysate and blood samples suggest that reflex endocrine response of the adrenal gland during hypovolemic hypotension significantly exceeds sympathetic neural response.