To verify the action of heparin on peritoneal transport, we selected 20 patients on acute peritoneal dialysis and performed two 2-hour cycles with 2,000 cm<sup>3</sup> of a 1.5% solution, adding 2,000 units of heparin to the second cycle. The patients were also randomized into 2 groups: group A, adding 1.5 mg gentamycin/kg to the dialysate of cycle I (without heparin), and group B, adding the same dose of gentamycin to cycle II (with heparin). At the end of each of the two cycles blood and dialysate were drawn for urea, creatinine, glucose, proteins and gentamycin levels, using peritoneal clearances of urea and creatinine, glucose absorption and net protein loss to compare cycle I with cycle II. We found that the peritoneal transport of creatinine and urea was improved (p < 0.02; p < 0.05) and glucose absorption increased (p < 0.01) with heparin, without any significant change in protein loss. Contrary to common belief, heparin in a 1,000-U/1 dose improved the absorption of gentamycin from the dialysate (p < 0.01).