1. The effects of isolation on systolic blood pressure, heart rate and fluid and electrolyte balances were studied in rats with a congenital inability to synthesize vasopressin (Brattleboro strain) and in the normal parent strain (Long Evans).
2. There was no significant difference between the systolic blood pressures of Brattleboro rats and Long Evans rats while the animals were housed in groups, although the heart rates of the Brattleboro rats were significantly higher.
3. After S days of isolation in metabolism cages, systolic blood pressure was significantly increased in the Long Evans rats, but not in the Brattleboro rats.
4. Since there were no significant differences between the fluid and electrolyte balances of the two groups after 5 days of isolation, it is unlikely that the hypertension in the Long Evans rats was attributable to a renal action of vasopressin.
5. It is possible that vasopressin was involved in the development of isolation-induced hypertension by virtue of its pressor effects. Alternatively, the failure of Brattleboro rats to develop hypertension may have been due to some abnormality in these animals other than the lack of vasopressin.