Twenty-two patients, aged 33-72 years, with uncomplicated essential hypertension were given sequential incremental intravenous infusions of sodium nitroprusside, each of 10 min duration, to examine the determinants of the vasodepressor response. Changes in mean arterial pressure (delta MAP), heart rate (delta HR), and plasma norepinephrine (delta nc) were determined at the end of each infusion period. The slopes of the dose-response curves obtained were directly proportional to predrug blood pressure (p less than 0.0001) and inversely proportional to baroreflex sensitivity as measured by the slope of the delta HR vs. delta MAP relationship (p = 0.0007). Baroreflex sensitivity was in turn inversely proportional to, and approximately equally dependent on, predrug blood pressure and age (p = 0.0116). Thus, the slopes of the dose-response curves were determined by both predrug blood pressure and patient age. The relationship was such that predrug blood pressure accounted for approximately 75% of the variability in the slope of the dose-response curve, and age for only 25%. This suggests that the age-related reduction in baroreflex sensitivity is mechanistically different from the hypertension-related reduction and is of less importance in modifying homeostatic responses to vasodilation.