It has previously been shown that stimulation of an adrenergic system in the central nervous system (CNS) will depress the increase in plasma corticosterone (B) following surgical stress. Two aspects of hypothalamo-pituitary-adrenal function, namely adrenal compensatory hypertrophy (ACH) and plasma B response to ether stress, were studied to determine the influence of this adrenergic component. Phenoxybenzamine, an α-adrenergic blocking agent, chronically implanted into the lateral ventricle produced an augmented response to ether stress, whereas norepinephrine (NE), dopamine (DA), and propranolol did not. The augmented response was similar to that observed following the daily administration of reserpine (0.25 mg/kg, s.c). The response to this dose of reserpine was blocked by the simultaneous administration of a low dose of dexamethasone (14 µg/kg, s.c). Augmentation of the stress response was not produced by larger doses of reserpine but did occur with lower doses. The effects of the treatments on ACH were not consistent with those observed on the stress response, suggesting that different mechanisms are involved. The data support the concept of an α-adrenergic component in the CNS which serves to inhibit the plasma B stress respon