Background:Previous studies aimed at developing an artificial placenta have had limited success. We hypothesized that the introduction of a high-performance membranous oxygenator to a pumpless artificial placenta could prolong the survival time of premature lambs.Methods:Immediately after delivery of the fetuses, the umbilical vessels were cannulated and connected to the pumpless artificial placenta. Both the fetuses and the circuit were submerged in a warm saline bath.Results:Five fetuses survived for 18.2 ± 3.2 (mean ± SEM) h after attachment to the artificial placenta, which maintained fetal circulation. Circuit blood flow was positively correlated with mean arterial pressure and negatively correlated with blood lactate levels. Milrinone administration transiently decreased lactate levels, although dopamine administration unexpectedly induced a marked increase in the lactate levels despite an elevated arterial pressure and improved circuit blood flow.Conclusion:We prolonged the survival of fetal lambs using a high-performance membranous oxygenator with a small priming volume. The increased systemic resistance induced by vasoconstrictors may increase the circuit blood flow excessively, resulting in circulation failure in systemic organs; therefore, vasodilators may be more useful than vasoconstrictors for maintaining organ blood flow within this circuit.