1. Factors contributing to the maintenance of resting arterial blood pressure in sham-operated and adrenalectomized rats were assessed 14 weeks after operation; some animals were given 1% NaCl to drink throughout, and some animals were given sodium-free water instead of 1% NaCl for 2 days before the experiment.

2. Inhibition of the renin-angiotensin (ANG) system (with captopril) and of the cardiovascular actions of vasopressin [with 1-(β-mercapto-β,β-cyclopentamethylenepropionic acid), 8-d-arginine vasopressin (d(CH2)5DAVP)] had no hypotensive effects in sham-operated rats under any conditions, but antagonism of nicotinic ganglionic transmission (with pentolinium) caused a prompt fall in blood pressure. During combined administration of captopril, d(CH2)5DAVP and pentolinium to sham-operated rats, there was no significant recovery of blood pressure.

3. Adrenalectomized rats drinking 1% NaCl showed a marked hypotension and tachycardia in response to captopril, but d(CH2)5DAVP had little effect, whether administered before or after captopril. Addition of pentolinium rendered these animals more hypotensive man the sham-operated rats.

4. Adrenalectomized rats drinking sodium-free water for 2 days were hypotensive and blood pressures were lowered further by d(CH2)5DAVP or captopril under these conditions; there was no accompanying tachycardia.

5. The results indicate that, in chronically adrenalectomized rats drinking 1% NaCl, blood pressure is maintained by autonomic nervous and renin-ANG systems with no discernible contribution from the peripheral cardiovascular actions of vasopressin. However, the latter contribute importantly to the support of blood pressure when the adrenalectomized rats are rendered hypovolemic and hypotensive by substitution of sodium-free water to drink.

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