1. Male Wistar rats develop systolic arterial hypertension when housed in glass metabolism cages. The present experiments were designed to investigate the involvement of the adrenal glands in this form of hypertension. 2. Rats were bilaterally adrenalectomized and maintained by either salt supplementation (1% sodium chloride solution instead of tap water to drink) or steroid replacement (corticosterone solution in the drinking water). 3. Adrenalectomized rats treated as above did not develop hypertension in response to isolation, whereas sham-operated rats (drinking either 1% saline or tap water) did. 4. Hypertension in the sham-operated rats was not accompanied by a renal retention of sodium and water. 5. It is concluded that increased adrenal activity is involved in the development of isolation-induced hypertension, but not by causing a fluid retention and hence volume expansion. The relative contributions of adrenal medullary and cortical activity remain to be determined.