An avcothanized vinyl sheet consisting of a Pall filter, 2 pressure monitoring lilnes, 2 heparinization lines, and a roller pump was used for transapical left ventricular bypass in 36 calves and 11 patients. Blood was returned to the abdominal or ascending aorta or carotid artery. Prolonged left ventricular flow unloading was carried out. All 31 calves that underwent left ventricular bypass and 5 calves that underwent left ventricular bypass and 5 calves that underwent biventricular bypass survived the scheduled period of bypass of up to 7 weeks except for 2 calves that died of accidental air embolism and 2 that were deliberately sacrificed due to infection. Local heparinization for 6 days followed by oral anticoagulation to maintain the prothrombin time at 150-200% of normal were effective in keeping the animals from bleeding and preventing thromboembolism. Eleven critically ill patients with severe heart disease were supported with left or biventricular bypass for periods of up to 172 hours. Four patients ultimately recovered, and 3 others were successfully weaned from bypass and expired 1-4 months later.