Protamine reversal of heparin anticoagulation occasionally induces the release of thromboxane into plasma with catastrophic pulmonary hypertension. To examine the site of neutralization, we labeled protamine sulfate with 111In and compared activity scans after administration of labeled protamine in unheparinized and heparin-anticoagulated sheep. Protamine administration in sheep without prior heparinization did not cause thromboxane release, pulmonary hypertension, or significant leukopenia, and 111In-protamine was rapidly cleared from the lungs (half time 0.48 +/- 0.08 min). Neutralization of heparin anticoagulation by labeled protamine produced elevated plasma thromboxane, pulmonary vasoconstriction, leukopenia, and prolonged pulmonary clearance of 111In-protamine (half time 3.32 +/- 0.43 min). In rats, protamine reversal of heparin anticoagulation did not induce either thromboxane synthesis or pulmonary hypertension, and 111In-protamine cleared rapidly from the lungs. Thus the ovine heparin-protamine reaction produces concomitant pulmonary sequestration of heparin-protamine complexes, thromboxane release, and pulmonary vasoconstriction; this did not occur in the rat. The lung specificity of the reaction and interspecies differences suggest that ovine pulmonary intravascular macrophages may be activated by heparin-protamine complexes to release thromboxane and provoke acute pulmonary vasoconstriction.