The tissue uptake of 3H-aprotinin was studied in anesthetized cats during acute myocardial ischemia (MI) 1 an 2 hr after injection of the tracer. Several tissues exhibited a rapid uptake of the protease inhibitor. Kidney, lung and liver demonstrated the greatest uptake with tissue/plasma ratios of 1.6 to 4.8. Spleen, adrenals, intestine, heart and pancreas exhibited tissue/plasma ratios of 0.28 to 0.58, whereas abdominal aorta, skeletal muscle and omentum had tissue/plasma ratios below 0.17. The rate of clearance of 3H-aprotinin from cat plasma was unaltered by myocardial ischemia. Although ischemic myocardial tissue took up less aprotinin than the non-ischemic myocardial tissue in the same hearts, ischemic tissue accumulated significant amounts of aprotinin relative to the non-ischemic region (82%). These data show that aprotinin reaches ischemic myocardial tissue during the first 2 hr of acute ischemia and would be available to antagonize some of the proteases which may be liberated during these critical early hours of the ischemic process.