In 24 patients on regular hemodialysis the serum levels and the half-life of dextran 40 were determined after single or repeated infusion of dextran 40 (Rheomacrodex¿). One group of hemodialysis trainees had a dextran 40 half-life of 2237 +/- 447 min. In contrast, a second group, consisting of hospital dialysis patients, had a dextran 40 half-life of 4283 +/- 510 min, whereas patients with normal renal function had a half-life of 573 +/- 183 min. The difference in dextran half-life between the two groups of hemodialysis patients may be explained by the better residual renal function of our hemodialysis trainees. During hemodialysis no change in serum dextran levels could be observed. Intermitten hemodialysis, also, had no influence on the exponential elimination of dextran over a period of 6.25 days. Since hemodialysis by itself has no effect upon dextran elimination, the dosage fo dextran 40 has to be adjucted to the prolonged elimination time only, disregarding dialysis. After the fourth infusion of 50 g dextran 40 (twice a week 500 ml of 10% dextran 40) serum dextran levels exceeding 50 g/l and cutaneous bleeding were observed. To avoid these complications an initial saturation dose is suggested, followed by a maintenance dose adjusted to the impaired renal function.