The hemodynamic response to reductions in systemic oxygen availability serves to redistribute blood flow and maintain vital organ function. The efficacy of this response depends on the degree to which hypoxia alters the function of the vascular tissues themselves. In this study we have evaluated these effects in rats exposed to 10% oxygen for 0 (control), 12, and 48 h and for 48 h followed by 12 h of normoxic recovery. In aortic segments from each group, the cumulative concentration response relationships were constructed for phenylephrine and KCl. Maximum tension generated during activation by these agents was reduced after both 12 and 48 h of hypoxic exposure. After 48 h of hypoxia, the maximum tension during activation by phenylephrine was 0.46 ± 0.04 vs. 1.31 ± 0.09 g/mg dry wt for the control group (P< 0.05 for difference). The maximum tension during activation by KCl was similarly affected (0.32 ± 0.02 vs. 0.98 ± 0.06 g/mg dry wt, 48 h of hypoxia vs. control, respectively; P < 0.05 for difference). Exposure to hypoxia did not alter the EC₅₀ for either agent. Twelve hours of normoxic recovery did not fully restore contractility after 48 h of hypoxia. In aortic rings from control rats, endothelial removal enhanced contraction, whereas, in rings from rats exposed to hypoxia, removal of the endothelium was associated with a decrease in maximum tension. Prolonged exposure to hypoxia results in impairment of systemic arterial smooth muscle contractility. This is partly compensated by the release of vasoconstricting substances from the endothelium.