Oxygen sensing in vascular smooth muscle offers a means of metabolic—contraction coupling whereby blood vessel calibre may be regulated according to the local tissue perfusion requirements. Thus, resistance vessels from many vascular beds respond to acute hypoxia with a reduction in tone perhaps to maintain organ perfusion in the face of increased metabolic stress. However, in pressurised small mesenteric arteries of the rat prestimulated by depolarisation, more prolonged hypoxic episodes result in vessel reconstriction to near pre—hypoxic diameters (Shaw et al., 2003). In this study we explore the role of the endothelium in these responses. Small mesenteric arteries isolated from male Wistar rats (humanely killed by stunning and cervical dislocation) were pressurised to 60 mmHg and superfused with HCO₃—buffered physiological saline gassed with 95% air,5% CO₂. Addition of High K solution produced a tonic constriction (170±13 μm to 58±14 μm, n=3, mean±sem) that was significantly reduced by 58±12% (students t test, p<0.05) by the addition of the endothelial—dependent agonist carbachol (10μM). Constrictions to high K solution could be maintained for up to 2 hours in normoxia and the carbachol—induced dilation after this time was also retained (reduction in diameter of 46±4%). In a separate series of experiments, introduction of hypoxia (taken as superfusate pO2<10mmHg for 10 mins) by gassing with 95% N2/5%CO*2 initially dilated high K pre—constricted arteries to 38±5% of normoxic diameter (n=4). When hypoxia was maintained for > 60 mins, this was followed by a reconstriction (to, after 120 mins, 70±5% of high K normoxic diameter). The relaxation to carbachol, however, was ablated after 2 hours hypoxia. As reported previously (Shaw et al., 2000), the nitric oxide synthase inhibitor L-NNA (50μM) prevented any dilation of normoxic high K—stimulated mesenteric arteries to carbachol (n=8). Moreover, in the presence of L—NNA, these preconstricted vessels only dilated to lowered oxygen (after 60 mins hypoxia diameter was 65±4% of high K in normoxia) and did not show any reconstriction even after 2 hours hypoxia (diameter 59±7% of normoxic high K). These results suggest that in pressurised rat mesenteric arteries chronic hypoxia impairs endothelial—derived nitric oxide release and this may account for the reconstriction observed in depolarised vessels in the continued presence of lowered oxygen.