The xenon washout technique and the renal blood flow response to vasoactive agents or alterations in sodium intake were used to characterize the effect of aging on the renal vasculature in 207 normal human subjects ranging in age from 17 to 76 years. A highly significant, progressive reduction in the mean blood flow, the rapid-component flow rate, and the percent of flow into the rapid-flow (cortical) compartment accompanied advancing age. Because ¹³³Xe measures flow per unit tissue mass, the results indicated a larger reduction in flow than in mass--the anticipated finding if flow reduction is primary in the genesis of atrophy. Age also reduced the vasodilation consequent to administration of acetylcholine or a sodium load; this finding is consistent with a fixed lesion of the vessels. Responses to angiotensin were not modified by age. Thus, offsetting factors of increased ratio of wall to lumen thickness and smooth muscle atrophy are precisely matched. The findings in this study agree with earlier hypotheses based on morphology that suggest a primary vascular process in the development of age-related renal changes.