Serum sulfate concentrations may reach five to ten times normal in renal failure patients dialyzed on a sorbent cartridge system, and these patients have elevated alkaline phosphatase levels suggesting an increased incidence of renal oseodystrophy. We studied the effect of adding sulfate on ionized calcium (Ca2+) in human serum in vitro and in rat serum in vivo. K2SO4 or Na2SO4:NaCl mixtures were added to aliquots of serum from normal subjects to reproduce the observed biologic range of sulfate concentrations up to 10 mmol. Serum Ca2+ concentration was found to decrease linearly as serum sulfate concentration increased, for each subject. The weighted mean slope estimates of the effect of sulfate on ionized calcium in two experiments were -.0197 and -.0181. Rats were infused through the inferior vena cava with 2 mL of either 200 mmol NaCl (N = 5) or 100 mmol Na2SO4 (N = 6), after ligation of the renal arteries and veins and withdrawal of 2 mL blood for baseline studies. The animals were killed by exsanguination from the aorta after a five-minute equilibration period. In rats administered NaCl, no difference in Ca2+ or sulfate concentration was found between pre- and postinfusion sera. In the Na2SO4 treated rats, however, a significant mean increase of 0.635 mmol (p less than .005) in serum sulfate concentration was associated with a significant mean decrease of -0.062 mmol (p less than .01) in serum Ca2+ concentration. We conclude that the acute in vitro and in vivo addition of sulfate results in a decrease in serum Ca2+ concentration. Thus, hypersulfatemia, which is present chronically in patients on sorbent dialysis systems, may contribute to elevated alkaline phosphatase levels in these patients.