The newborn infant requires more heparin per kg body weight than the adult to achieve similar heparin plasma levels. Possible mechanisms include altered heparin pharmacokinetics and/or a decreased expression of anticoagulant activity of heparin in new-born plasma because of low levels of antithrombin III (AT-III). We measured the pharmacokinetics and the anticoagulant activity of heparin in the pig (AT-III level: 100%), in the piglet (levels of AT-III: 50% of adult) and the piglet given exogenous porcine AT-III. All pigs were bolused with 125I-heparin (25 or 100 units/kg) and blood samples collected for the measurement of 125I-radioactivity, and antifactor Xa activity. The half-life of 125I-heparin was dose-dependent and similar in pigs and piglets; however, the volume of distribution was greater in the newborn resulting in an increased total clearance compared to the pig. The anti-factor Xa activity disappeared earlier in the piglet than in the pig. Both the kinetics and the absolute recovery of anti-factor Xa activity were normalized to pig values (after correction for different volumes of distribution) when the piglets were infused with exogenous AT-III. Thus apparent heparin resistance of the newborn is due to both an increased volume of distribution and the low AT-III level which limits the measurement of the anticoagulant activity of heparin in conventional anti-factor Xa assays.