1. Neurosecretion of peptides from superfused neurohypophyses in vitro was inhibited by dopamine.
2. This inhibition was dose-dependent.
3. Intravenous injection of the dopamine agonist, bromocriptine, lowered blood pressure in spontaneously hypertensive rats within 15 min.
4. Saralasin or captopril also lowered blood pressure of spontaneously hypertensive rats, but progressively over a period of 3 h.
5. The results suggest that dopamine and angiotensin have opposite effects on the neurosecretion of vasopressin.
6. Vasopressin appears to be involved in maintenance of blood pressure in the spontaneously hypertensive rat but is apparently not the only factor.