Abstract In this study we have quantified the potency of three cyclo-oxygenase inhibitors, indomethacin, flurbiprofen and aspirin to reduce the vasoconstriction of isolated strips of human umbilical artery (HUA) to changing the oxygen-tension ( PO 2) of the bathing medium. The inhibitory potencies (IC 50) of indomethacin and flurbiprofen were similar (IC 50 = 2.1 × 10 −9M and 7.6 × 10 −9M respectively) while aspirin (IC 50 = 2.5 × 10 −5M) was approximately 12, 000 fold less potent than indomethacin. At the physiological PO 2 of 15mmHg the isolated HUA was found to have an inherent cyclo-oxygenase induced tone since either reducing the PO 2 from 15 to OmmHg or adding cyclo-oxygenase inhibitors caused vasorelaxation. O 2-induced contractions were partly dependent on extracellular Ca 2+: nifedine and Bay K 8644 inhibited and enhanced oxygen-induced contractions, respectively, but did not have a significant effect on calcium-dependent 5-hydroxy-tyrptamine-induced (5-HT) contractions. Therefore cyclo-oxygenase products and 5-hydroxytryptamine-induced contractions of the HUA may both utilise extracellular calcium but through different processes.