The maintenance of adequate maternal-fetal circulation is essential for fetal growth. The circulation appears to be controlled by local prostanoid synthesis which may affect the vascular tone. With a view to examining this possibility, the present study was designed to evaluate the umbilical circulation serially in vivo by means of pulse doppler flowmetry. After delivery, the cord vasculature and the placenta were studied in vitro for the synthesis and release of prostacyclin (PGI2) and thromboxane A2 (TxA2). The resistance index (RI) showed a significant negative correlation to the birth weight (n = 15, r = - 0.70, p less than 0.01) and a positive correlation to the maternal arterial pressure (r = 0.56, p less than 0.05). PGI2 synthesis was highest in the umbilical artery, then in the umbilical vein, and lowest in the placenta. TxA2 tended to be higher in the umbilical artery of light-for-date neonates, showing a negative correlation with the birth weight (r = - 0.56, p less than 0.05). RI showed a significant correlation with the PGI2/TxA2 ratio. These results indicate that the balance between PGI2 and TxA2 influences cord circulation and in turn fetal growth.