It was proposed that the diminished renal function observed with advancing age could be due to a lower concentration of essential ions within the organ or an accumulation of ions which could produce cellular toxicity. This investigation reports the effect of age (3, 6, 12, 24 and 28 months) on the concentrations of magnesium, calcium, zinc, copper and iron in renal homogenates of Fischer 344 strain rats. Samples of renal tissues were homogenized in a solution containing 5 p. 100 La + 25 p. 100 HNO3. The samples were digested, centrifuged and assayed by atomic absorption spectrophotometry. With age, the magnesium concentration decreased (P less than 0.05) while the calcium and copper concentrations remained unchanged, and the zinc concentration decreased slightly while the iron concentration markedly increased (P less than 0.05). In summary, these data and other data obtained from similar studies of the brain and heart, suggest that the changes observed with age are unique to the kidney. Our results suggest that changes in ionic concentration in the kidney with respect to age could contribute to the diminished function observed in that organ with senescence.