1. The vasoconstrictor and vasodilator activity of cultured bovine pulmonary artery endothelial cells was measured to determine how exposure to different partial pressures of O2 [PO2 142,42 and 15 mmHg (18.9, 5.6 and 2 kPa)] affects the production of endothelial-derived relaxing and constrictor factors. 2. A de-endothelialized rat aortic ring [maintained at a PO2 of 142 mmHg (18.9 kPa)] was used to bioassay the effluent from a perfused column of bovine endothelial cells grown on microcarrier beads. The endothelial cells were stimulated by 10(-7) mol/l bradykinin given for 1 min at 12 min intervals. 3. At the start, middle and end of the experiment the bovine endothelial cells were exposed to a PO2 of 142 mmHg (18.9 kPa) and when stimulated by bradykinin the perfusate caused respectively a 70 +/- 4%, 63 +/- 6% and 63 +/- 6% (mean +/- SEM) relaxation of an aortic ring which had been pre-contracted by 10(-6) mol/l phenylephrine. At a PO2 of 42 mmHg (5.6 kPa) the relaxation induced by the cells was not significantly altered, but this tailed to zero after 26-38 min exposure of the cells to a PO2 of 15 mmHg (2 kPa). 4. These responses were unaltered by the presence of 10(-5) mol/l indomethacin, suggesting that prostacyclin is not a significant vasodilator in this system. 5. Reduced production of endothelium-derived relaxant factor rather than production of a constrictor factor, or a direct effect on the smooth muscle, may be involved in pulmonary hypoxic vasoconstriction.