In this study, we compared results obtained in senescent rats with young rats given an equivalent acid load. We examined the renal changes by giving equivalent acid loads for 48 h to both 6- and 24-mo-old rats. The basal excretion of ammonium was the same in both groups, whereas titratable acids, phosphate, and Ca2+ excretions were increased in the senescent animal. After administration of the acid load, ammonium, phosphate, Ca2+, and titratable acid excretions increased in both age groups, but there were greater absolute increases in ammonium and titratable acid excretions in the young rats. The total acid excreted by the 24-mo rats was reduced 50 (day 1) and 25% (day 2) compared with the young rats, which was reflected by the more severe acidosis in those animals. The portion of total acid excreted as titratable acids in senescent animals was also increased during acidosis when compared with the young animals. In isolated proximal tubule brush-border membrane vesicles, acidosis increased Na+-H+ exchange and decreased Na+-dependent phosphate transport in both age groups. We also found that the basal activity of the Na+-H+ exchanger was not changed with age but the Na+-dependent phosphate transporter was less in the 24-mo rat. The results suggest that physiological regulation of these renal processes remains intact in the aged rat but the responses may be reduced or delayed in the senescent animal.