Intravenous protamine reversal of heparin anticoagulation may cause adverse hemodynamic side effects, but little is known about protamine's tissue distribution, circulating half-life (t/2), and excretion. The latter were assessed by examining 125I Bolton-Hunter (125I BH) radiolabeled protamine kinetics in a rat model. Three groups were studied: Group I controls (n = 5) received intravenous 125I BH label alone; Group II (n = 10) received intravenous 125I BH radiolabeled protamine (0.15 mg/100 g); and Group III (n = 10) received intravenous heparin (15 IU/100 g) followed by intravenous 125I BH radiolabeled protamine (0.15 mg/100 g). Five animals in each group were killed at 3 min, and tissue radioactivity was quantitated. An additional five animals each in Groups II and III were followed up for 60 min to determine protamine's circulating t/2 and its renal excretion. The lungs, heart, and kidneys, compared with other organs, retained the most 125I BH radiolabeled protamine per gram tissue at 3 min. Retention of 125I BH radiolabeled protamine (Groups II & III) was greater (p < 0.05, Kruskal-Wallis) than control 125I BH label alone (Group I). Higher tissue 125I activity was observed in Group II than in Group III rats, suggesting that tissue retention of protamine was greater in the absence of prior heparin administration. Circulating t/2 was shorter (18 vs. 24 min) and urinary protamine 125I excretion was higher (34 vs. 24%) in Group III than in Group II, respectively, suggesting more rapid renal clearance of protamine in the presence of heparin.(ABSTRACT TRUNCATED AT 250 WORDS)