In the search for a more acceptable route of heparin administration that can also be used for long-term treatment, we evaluated the bioavailability and antithrombotic activity of intraduodenal administration of unfractioned heparin (UFH) in rats. A radioiodinate derivative of UFH was administered intraduodenally in rats in conjunction with unlabeled UFH. We found that radioactivity increases very rapidly in plasma, as well as on the surface of aortic and caval segments, so that peak radioactivity was already achieved within 5 min of drug administration. Subsequently, plasma radioactivity declined rapidly, although 1.5-2.5% of the total radioactivity administered was still circulating 3 h after drug administration. The plasma anti-Xa activity was much lower and longer lasting than expected from the radioactivity counts in both peripheral and portal blood, and its level was very similar in the two circulatory districts, never exceeding 023 U/ml. This suggests that extensive degradation of the drug already occurs during its gastrointestinal absorption. Nevertheless, in a stasis-induced venous thrombosis model, intraduodenal UFH prevented thrombus formation in a dose-dependent way (ED50, 2000 IU/kg). The maximum antithrombotic effect was observed when the drug was administered 30-60 min before ligature of the vena cava, and a 40% reduction of thrombus weight was still present at 180 min. Since antithrombotic kinetics does not match the kinetics of either the plasma or vessel-bound radioactivity but approaches what is found in anti-Xa activity, the antithrombotic activity of oral heparin may be dependent on the release of unlabeled endogenous glycosaminoglycans deposited in the vessels.