A small, 257 g centrifugal pump was tested as a left ventricular assist device (LVAD) in sheep given a myocardial infarction. Pump performance, hemolysis, end organ function, weaning, explant procedure, and the incidence of thromboemboli at autopsy were studied over intervals of 1 to 44 days. Twelve sheep were given acute myocardial infarction by ligation of the anterior descending coronary artery and 11 had insertion of the AB-180 Circulatory Support System (CSS). One sheep served as a control for the space occupying effects of the pump in the left chest. Inflow was from the left atrium and outflow was to the descending thoracic aorta. Heparin (57–83 U/ml) in sterile water was infused into the pump at a rate of 10 ml/hr. Pump flows of 1–5.7 L/min were tested. The AB-180 CSS supported 73.5% of the total cardiac output (pump + heart) of 3.89 L/min, with a mean arterial pressure of 86 ± 7 mmHg at a pump speed of 4,162 ± 276 rpm immediately after implant. Hemolysis was <10 mg/dl and activated partial thromboplastin time (aPTT) values were in the normal range for sheep (<52 sec) after 48 hr of pumping. Liver enzyme concentrations returned to normal within 2 weeks. There was no evidence of thrombocytopenia. No signs of infection were present during assist and none was found at autopsy. The device was successfully removed three times without the use of pressor agents or blood transfusion. Alarm systems performed appropriately. During the 106 days of cumulative pumping, two sheep showed small (<1.5 cm) renal infarcts. Both were associated with intervals of pump stasis. The AB-180 CSS pump was easily implanted into the left chest without the use of cardiopulmonary bypass. It appears to have a low thromboembolic potential in sheep, without the need for large doses of heparin to elevate aPTT values. This characteristic may ameliorate the excessive bleeding seen clinically with current LVAD systems used for post cardiotomy cardiogenic shock, which require anticoagulation with heparin. The small size and weight of the device permit implantation within the chest and allow chest closure. This may reduce the incidence of infection associated with temporary left ventricular assist and an open sternum.