The poor prognosis of certain cardiac abnormalities detected prenatally by echocardiography has led some investigators to consider intrauterine cardiac surgery. Investigation into the efficacy of open cardiac procedures in-utero will require techniques for safe and effective fetal extracorporeal circulation and myocardial protection. We performed cardiopulmonary bypass in 8 fetal lambs to assess the feasibility of fetal cardiopulmonary bypass. Four fetuses underwent studies at 37 degrees C (Group I) and 4 at 25 degrees C (Group II). The aorta was clamped and cold crystalloid cardioplegia administered. Perfusion was carried out for 10 minutes each at high (95 +/- 18 cc/kg/min), moderate (67 +/- 10 cc/kg/min), and low (49 +/- 8 cc/kg/min) flow rates while hemodynamic and blood gas measurements were made. Total time on bypass averaged 57 min in Group I and 75 min in Group II. Four fetuses were successfully weaned from bypass following the study period. Fetal pO2 and oxygen saturation was very low at all flow rates in Group II and at low flow rates in Group I, indicating poor function of the placenta as an oxygenator at 25 degrees C and at low flow rates during normothermia. Lambs undergoing bypass at 37 degrees C had a progressive rise in pCO2 levels as flow decreased, while pCO2 was relatively normal at all flow rates at 25 degrees C. These studies serve as a starting point for the development of techniques to allow intrauterine correction of experimentally produced fetal cardiac lesions.