This investigation was undertaken to define the "adequate" sodium concentration in the dialytic fluid allowing to maintain a stable plasma effective osmolality during dialysis. Isonatric dialysate is shown to miss this aim by inducing a predictable postdialytic hypernatremia. To avoid this effect a new approach was made. 17 clinically stabilized patients, previously dialyzed over a period of at least 2 years with a dialysate sodium concentration of 133 mEq/l, underwent dialysis with the "adequate" sodium concentration in the dialysate for over 3 years. During dialysis cramps, headache, hypotension, hypertensive crises and postdialytic weakness were reduced in frequency and nearly disappeared. No deterioration in blood pressure control occurred and improvement in some general parameters (hematocrit, glucose and insulin metabolism, well-being) was reported after prolonged treatment.