Plasma concentrations of calcium often increase during hemodialysis. To evaluate the mechanisms involved, total (Ca) and ionized (Ca++) calcium, protein, pH, Pco2, and sodium (Na+) were measured in plasma (P) at the beginning and end of 17 six-hour coil dialyses using bath with 5.27 ± 0.04 mg/100 ml of Ca++. Simultaneously, ultrafiltrates (UF) collected directly from coils (after isolating and draining the cannister) were analyzed for Ca, Ca++, and Na+. Increases in plasma Ca reflected mainly increases in protein-bound calcium (secondary to bath hemoconcentration of protein and increased binding per gram of protein) without an increase in plasma Ca++ or complexed Ca. With increases in plasma pH and protein concentration, (UF/P Na+)2 decreased only slightly suggesting minimal changes in net protein anionic concentration and Gibbs-Donnan effects.