Changes in kidney function were studied in anesthetized female WAG/Rij rats 6, 18, and 30 mo old. The growth in kidney size measured between 6 and 18 mo and 18 and 30 mo was due to enlargement of the glomeruli and lengthening of the proximal tubules, without significant changes in the number of nephrons. Glomerular filtration per gram kidney was unaffected between 6 and 18 mo but diminished 27% between 18 and 30 mo. This reduction was accompanied by an intrarenal redistribution of individual filtration rates for the superficial and deep nephrons. Fractional excretion of sodium, potassium, calcium, and magnesium declined sharply between 6 and 18 mo but did not change further between 18 and 30 mo, whereas the fractional excretion of amino acids and glucose remained constant between 6 and 18 mo but rose in the 30-mo group. Taken together, these observations indicate that, in the rat, kidney aging cannot be reduced to a loss in the number of functional nephrons but is the result of several differential and specific processes.