Mice of the C57BL/6J and CBA/HT6J inbred strains and their F¹ hybrid were investigated at 67, 202, 463, and 711 days of age to determine how genetic differences affected aging patterns of renal function and morphology. The three genotypes differed significantly in their body and kidney weights and their kidney to body weight ratios. These differences reflected functional capacities, and renal hypertrophy appeared to be an attempt to compensate for functional losses with age. The glomerular filtration rate (GFR) decreased in both inbred strains with age but remained constant in the F¹ hybrid mice. There was a significant strain difference in GFR, but this difference disappeared when the GFR was considered in relation to the kidney weight. Thus the strain difference appeared to reflect differences in ability to compensate for loss of function by increasing kidney size. Osmolarities and sodium clearances decreased with age and increased with renal hypertrophy, but plasma levels were constant. Histopathological changes with age appeared to be correlated with functional changes.