Activated partial thromboplastin time (aPTT) during heparin infusion in 10 patients on regular dialytic treatment was evaluated. At end-dialysis (3 hours) plasma heparinization ranged between 2000-4000 IU and from the clinical point of view dialytic performance was good. At end-dialysis aPTT was 53 +/- 14 seconds: this value is under the prescribed aPTT prolongation (1.5-2 times baseline values) to achieve a therapeutic heparin plasma level. Physiopathological implications of this aPTT "resistance" to heparin infusion are studied.