SummaryThe placentas of eight near-term goats were perfused with blood containing C2H2. The adult goats breathed gas containing N2O. Catheters were placed in a maternal femoral artery and uterine vein. Knowledge of the perfusion rate and N2O and C2H2 concentrations in the maternal artery, uterine vein, umbilical arteries, and umbilical veins, permitted the calculation of the uterine blood flow.The uterine flow was calculated before and after a change in the rate of the umbilical perfusion. Fourteen such observations were made. In 13 cases the uterine flow changed in the same direction as did the umbilical flow (P < 0.005). The magnitude of the change in uterine flow indicated that the mechanism whereby the change was mediated had a gain of greater than 1.This work was supported by Grants PHS HE05499, HE06336, and HD2313 and by the Oregon Heart Association.The authors wish to acknowledge the assistance of Mr. J. E. Welch and Mr. R. Ackley in the conduction of the experimental protocols.