The effects of hemorrhage on plasma vasopressin levels and blood pressure were examined in conscious, age-matched spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rats. Graded hemorrhage was produced by bleeding the rats at 10 min intervals over a total period of 65 min, to produce cumulative blood losses equivalent to 0.5, 1.0, 1.5, 2.0 and 3.0% of body weight in each animal. Hemorrhage progressively lowered blood pressure and increased plasma vasopressin levels in both SHR and WKY. At cumulative reductions in blood volume equivalent to 1.0 to 3.0% of body weight, there were greater reductions in arterial pressure and greater increases in plasma vasopressin concentrations in SHR than WKY. Basal blood volume in SHR was 10% lower than in WKY. In SHR, the greater vasopressin response to hemorrhage may have been due in part to the greater fall in arterial pressure. Although the lower blood volume may have contributed to the latter, derangements in baroreceptor function may also have been important.