In 2 groups of 4 probands the pharmacokinetics of heparin were investigated by a physiologic method (factor Xa inhibition) and an isotope method (35S-heparin) following a single intravenous injection of 5000 IU heparin and a single subcutaneous injection of 10000 IU heparin respectively. Following intravenous administration the anticoagulant effect and radioactivity fall exponentially. The half-life is about 50 min and the distribution volume 3000 ml (factor Xa inhibition) and 3800 ml (radioactivity). With subcutaneous injection peak concentrations above 0.2 IR heparin/ml were measured by both methods 2-4 h after administration; 9 h after administration, no further factor Xa inhibition was detectable in 2 probands. In the light of these and previously published results, some practical aspects of the conduct of therapy and prophylaxis of thromboembolism with heparin are discussed.