Hypoxic vasoconstriction was investigated in isolated pulmonary and mesenteric arteries of the rat. Experiments were performed on large (approximately 2 mm pulmonary, approximately 0.8 mm mesenteric) and small (100-350 microns) arteries. Hypoxia [oxygen partial pressure (PO2) approximately 33 mmHg] elicited a biphasic response in arteries precontracted with prostaglandin F2 alpha (10 microM). A transient contraction reaching a peak within 2-3 min was observed in both large and small pulmonary and mesenteric arteries (phase 1). In pulmonary arteries, this was followed by a slowly developing contraction over 45 min (phase 2). In mesenteric arteries, there was no phase 2 but instead a profound relaxation. Mechanical disruption of the endothelium had no significant effect on phase 1 in preconstricted large pulmonary arteries but reduced phase 1 in small arteries by 40%. Phase 2 was abolished in both large and small arteries. Inhibition of endothelium-derived relaxing factor synthesis or cyclooxygenase pathways had no effect on either phase. Verapamil substantially reduced phase 1 but abolished phase 2. In conclusion, we have found a clear biphasic response to hypoxia in pulmonary arteries of the rat, but, in contrast to some previous reports, phase 1 was only partially dependent on the endothelium, whereas phase 2 was entirely dependent on the endothelium. Small and large arteries had qualitatively similar responses. These results are consistent with the involvement of at least two mechanisms for hypoxic vasoconstriction, one of which may involve release of an as yet unidentified endothelium-derived constrictor factor.