Vasopressin plasma concentrations have been measured in two groups of subjects, 13 moderate essential hypertensive patients without target organ damage and eight control normotensive subjects, before and after the assumption of the upright position, and intravenous infusions of hypotonic saline (0.45% NaCl, 0.25 ml kg−1 min−1 for 1 h) and hypertonic saline (100 mmol NaCl in 50 ml). Plasma vasopressin in recumbent baseline conditions was not significantly different in the two groups. Upright posture and hypertonic challenge augmented, while hypotonic saline reduced plasma vasopressin levels, which were not significantly different between the two groups. Plasma renin activity increased in the upright position, was reduced by administration of hypotonic saline and unaffected by hypertonic saline, with no differences between the hypertensives and normotensives. After hypertonic saline, urinary flow rate and urinary sodium excretion in the hypertensive group increased to values significantly (p < 0.05) higher than in normotensive subjects. In conclusion our study excludes significant alteration of vasopressin regulation in moderate uncomplicated hypertension. In hypertensives although the response of vasopressin to an osmotic load is preserved, the data suggest that the renal handling of the osmotic load may be altered.