Comparisons were made between responses evoked by noradrenaline (NA) in iliac artery rings from normoxic (N) rats and chronically hypoxic (CH) rats kept in 12 % O(2) for 3-4 weeks. At P(O(2)) of 100 mmHg, cumulative concentration-response curves (CCRC) to NA were greatly depressed in endothelium-intact (E+) rings, but not endothelium-denuded (E-) rings, of CH rats relative to N rats. However, CCRCs evoked by NA in E+ and E- rings during nitric oxide (NO) synthase inhibition were similar in N and CH rats. Reducing P(O(2)) to 55 mmHg depressed CCRCs to NA in E+ and E- rings of CH and N rats in the absence and presence of NO synthase inhibition. At P(O(2)) of 100 mmHg, CCRCs evoked by phenylephrine were comparable in E+ and E- rings of N and CH rats as were CCRCs for the relaxation evoked by isoprenaline, which were similarly rightward shifted by NO synthase inhibition. However, CCRCs evoked by the NO donor sodium nitroprusside were leftward shifted in E- rings of CH rats relative to N rats. Further, in the presence of the alpha(2) adrenoceptor inhibitor rauwolscine, CCRCs to NA were comparable in E+ rings of CH and N rats. Thus, the depressive effects of chronic hypoxia on NA-evoked contractions of iliac artery are additional to those of acute hypoxia. We propose that they reflect a facilitation of the contribution of NO to alpha(2) adrenoceptor-evoked relaxation that includes an increased sensitivity of the vascular smooth muscle of arteries from CH rats to NO. Experimental Physiology (2003) 88.4, 497-507.