Overview: renal physiology and pathophysiology of aging. Cross-sectional studies in humans have suggested that there is a progressive decline of renal function with age after 40 years. The decline in various functions (eg, tubular maximums, concentrating and diluting abilities, and acidification) tend to parallel the decreases in glomerular filtration rate (GFR) and renal blood (plasma) flow (RPF). Recent observations from the Baltimore Longitudinal Study of Aging suggest that not all individuals follow this pattern, and that, indeed, many show no decline and some even an increase in their renal function over time. Whether the observed decreases in renal function with aging are the results of intervening pathologic processes, eg, immunologic, infectious, and toxic injury and ischemia, or can be related to hyperperfusion and hyperfiltration with resultant glomerulosclerosis, or to some other relentless involutional process, remains unclear. The purpose of this report is to review the descriptive studies documenting the changes in renal morphology and physiology with age and to discuss what is known about mechanisms involved in these losses of renal substance and function. 