Cytoplasmic Ca2+ was measured in donors and 12 patients with chronic renal failure (CRF) on hemodialysis. Their basal platelet levels were approximately the same. With longer duration of the dialysis Ca2+ levels decreased. Thrombin-induced platelet production of Ca2+ in patients proved significantly lower than in donors, but the dialysis promoted a rise in both basal and thrombin-induced cytoplasmic Ca2+ levels. Greater concentrations of Ca2+ towards the end of the dialysis are supposed to result from reestablishment of its storage pools both in depositing granules and, primarily, in the dense tubular system.