We hypothesized that urinary excretion of calcium would be affected by both urinary excretion of acid and of total fixed anions. Calcium, magnesium and phosphorus utilization was examined in rats fed semipurified diets supplemented with approximately 0.4 mol sodium/kg diet or approximately 0.4 mol potassium/kg diet as chloride, sulfate, bisulfate, carbonate or bicarbonate salts in two studies. The ingestion of supplemental fixed anions (chloride, sulfate or bisulfate) increased urinary excretion of calcium, magnesium and phosphorus. It made no difference whether the anions were ingested as sodium or potassium salts. In Study 1, 80% of the variation in urinary calcium excretion could be explained on the basis of urinary excretion of sulfate, ammonia and total anions. In Study 2, 77% of the variation in urinary calcium excretion could be predicted on the basis of urinary excretion of total anions and sulfate. Although bone and plasma calcium concentrations were not responsive to these dietary changes, less magnesium was retained in bones of rats fed any of the supplemental salts.