The hypothesis that central alpha-1 adrenoceptors are inhibitory to the hypotension-induced secretion of vasopressin was tested by subjecting lambs that were instrumented for a long term to varying degrees of hypotension after intracerebroventricular injections of prazosin or placebo. Eight lambs in the 1st wk of life treated with intracerebroventricular injections of placebo had their mean arterial blood pressures decreased 14 and 21% by i.v. infusion of nitroprusside. Arginine vasopressin levels rose to 7.3 +/- 2.4 pmol/L only with the greater degree of hypotension. When the lambs were treated with intracerebroventricular injections of 1 micrograms/kg of prazosin, the blood pressures were decreased 13 and 23%, and the vasopressin levels were 15.4 +/- 16.6 and 27.5 +/- 20.3 pmol/L, respectively. A relationship was shown between the degree of hypotension and the plasma arginine vasopressin levels with both the placebo and prazosin, the slope being much steeper for the prazosin treatment (-1.11) than for the placebo treatment (-0.31). Plasma renin activity was increased a similar amount in both groups, and there was no change in plasma cortisol levels. We conclude that alpha-1 adrenoceptors in the brain are inhibitory to the secretion of arginine vasopressin. These results differ from observations in adult rats and dogs and may be accounted for by developmental or species differences.