We wished to determine the effect of prostaglandin E2 (PGE2) on the response of the ductus arteriosus to norepinephrine (NE) and whether any effect of PGE2 was influenced by O2 tension. The vessel was isolated from fetal New Zealand White rabbits and studied in vitro. The response to NE was assessed in terms of both sensitivity (pEC50) and maximum contractile response (MCR) as determined from cumulative concentration-response curves. PGE2 caused a concentration-dependent inhibition of ductal sensitivity to NE. This was maximal in nanomolar concentrations of PGE2, in which ductal sensitivity to NE was decreased by approximately 50 times. This action was only minimally potentiated by 3% O2 (simulating fetal O2 tension, PO2). PGE2 could also inhibit the MCR to NE, but this was entirely dependent on PO2. In 95% O2, PGE2 had no effect on the MCR to NE, whereas in fetal PO2, PGE2 decreased the MCR. This decrease was maximal in 1 nM PGE2, in which the MCR in 3% O2 was about one fifth the size of the MCR in 95% O2. In 1 nM nM PGE2, 10% O2 also largely abolished the inhibitory effect of PGE2 on ductal MCR to NE, indicating that this effect of O2 is also observed in the range of the physiologic increase of arterial PO2, tension that occurs at birth. PGE2 also inhibited ductal sensitivity to 5-hydroxytryptamine, histamine, and potassium. We conclude that physiologic and therapeutic concentrations of PGE2 inhibit sensitivity of the ductus arteriosus to certain vasoconstrictors and can inhibit the maximum response to NE in fetal but not increased PO2.