1. Plasma volumes and cardiovascular status were assessed in rats with a congenital deficiency in hypothalamic vasopressin (Brattleboro strain) and in the parent strain (Long Evans), in water-replete and water-deprived states.

2. Water-replete Brattleboro rats were not hypovolaemic; water deprivation (14 h in Brattleboro rats, 53 h in Long Evans rats) produced similar percentage reductions in plasma volumes in the two groups.

3. In the water-replete state, cardiovascular variables were similar in Long Evans and Brattleboro rats. Inhibition of ganglionic transmission (with pentolinium) or of the renin-angiotensin system (with captopril), separately, did not have a greater effect on blood pressure in Brattleboro rats than in Long Evans rats. Recovery from hypotension caused by pentolinium was characterized by large swings in blood pressure in both groups of rats. These pressor episodes were abolished by administration of captopril to Brattleboro rats. After administration of pentolinium and captopril to Long Evans rats there was a substantial, although intermittent, recovery in blood pressure that was abolished by an antagonist of the cardiovascular actions of vasopressin.

4. In the water-deprived state, blood pressures were similar in Long Evans and Brattleboro rats; both groups showed an elevation in diastolic blood pressure relative to the water-replete state. After administration of pentolinium, there was a more marked recovery in blood pressure than was seen in the water-replete state. Administration of Captopril alone had a slightly greater effect on blood pressure in Long Evans rats in the water-deprived, compared with the water-replete, state. However, in the former condition, Brattleboro rats showed a profound and progressive hypotension in response to captopril, indicating an indispensable role for the renin-angiotensin system in the maintenance of blood pressure in these animals during water deprivation.

5. Only when the renin-angiotensin system and neural activity were inhibited did vasopressin express its full, independent, pressor potential in Long Evans rats. However, evidence was obtained that vasopressin may exert important effects on cardiovascular regulation via neural mechanisms and through interactions with the renin-angiotensin system.

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