To evaluate whether hypoxia-induced increases in adrenal cortical (CQ) and medullary (MQ) blood flow (radiolabeled microspheres) occur secondary to hypoxia-induced secretory activity, pentobarbital-anesthetized ventilated dogs were pretreated with dexamethasone (DEX) to prevent adrenocorticotropic hormone (ACTH) and corticosteroid secretory changes or underwent unilateral adrenal denervation to prevent adrenal catecholamine secretory responses. In nonsurgically stressed dogs, DEX completely prevented increases in ACTH or corticosteroid levels during reduction of arterial oxygen content to 8 vol% but had no effect on hypoxia-induced doubling of CQ. In dogs in which adrenal oxygen consumption (VO2) was measured, DEX reduced VO2 by 50% without altering CQ. Unilateral adrenal denervation prevented hypoxia-induced increases in adrenal catecholamine secretion and MQ but had no effect on the CQ response. These results suggest that hypoxia-induced medullary vasodilation is associated with adrenal catecholamine secretory activity but that increases in CQ occur independent of secretory activity and likely represent direct vascular effects of hypoxia.