The aim of this study was to characterise the response to acute hypoxia in pulmonary artery rings isolated from rats exposed to chronic hypoxia for 2 weeks (CH) and following recovery in room air for 24 h (post hypoxic, PH). Large intrapulmonary artery (IPA) rings (internal diameter = 1.5 +/- 0.11 mm; n = 13) from CH and PH rats and age-matched controls were studied. These were precontracted with phenylephrine using standard organ bath procedures at an oxygen tension of 152 mmHg and subjected to an acute hypoxia stimulus (bubbling with 0% O2 giving Po2 = 7 mmHg or 2% O2 giving PO2 = 20 mmHg). Acute hypoxia-induced pulmonary vasoconstriction (HPV) consisted of a transient contraction, a relaxation and a sustained contraction over 30 min. Pulmonary vasoconstriction induced by 0% O2 was significantly reduced in IPA rings from the CH but not PH group compared with the response obtained from the control group. HPV induced by 2% O2 in IPA rings from CH and PH rats was not significantly different from that in control rats not subjected to chronic hypoxia. Mechanical removal of the endothelium or inhibition of nitric oxide (NO) synthase by L-NOARG (300 microM) reduced the contractile phases of HPV in IPA rings from control and CH rats. Carbachol-induced endothelium-dependent relaxation in phenylephrine precontracted IPA rings was significantly attenuated in the CH but not PH group. In conclusion, the present study demonstrates that HPV induced by 0% O2 in rat IPA rings was blunted in CH rats and restored following 24 h in room air, in parallel with changes in endothelium function.