Alterations in renal net acid excretion were studied in uninephrectomized dogs given 400 R total body irradiation. The early postirradiation (days 1-6) increase in acid excretion was the result of enhanced acid secretion rather than decreased base filtration and was related to a fall in Na+ and K+ excretion. The later postirradiation (days 7-11) decrease in net acid excretion was the result of diminished acid secretion. However, the decrease in acid secretion was not related to an increase in Na+ or K+ excretion, nor could it be attributed to an alteration in carbonic anhydrase activity. Rather the decrease in acid secretion appeared with an elevation in plasma Cl- and fall in plasma HCO3- suggesting an increase in NaCl reabsorption at the expense of the Na+ for H+ exchange.