Exposure to competitive mental tasks significantly reduced the urinary sodium and fluid excreted by young men with one or two hypertensive parents or with borderline hypertension. In this high-risk group, the degree of retention was directly related to the magnitude of heart rate increase during stress, suggesting common mediation by way of the sympathetic nervous system. Thus, psychological stress appears to induce changes in renal excretory functions that may play a critical role in long-term blood pressure regulation.