We examined the effects of hypoxic/ischemic stress on cerebral arteriolar responses to oxytocin in anesthetized piglets. Pial arteriolar diameters were measured using a cranial window and intravital microscopy. First, we evaluated arteriolar responses to topical application of oxytocin during normoxic conditions. We then determined whether 5–10 min of arterial hypoxia, ischemia, or asphyxia alters oxytocin-induced responses. Arterial hypoxia was produced by inhalation of 7.5% O<sub>2</sub>-92.5% N<sub>2</sub> for 10 min. Ischemia was achieved by increasing intracranial pressure for 10 min. Asphyxia was achieved by turning off the ventilator for 5 min. During normoxic conditions, oxytocin dilated pial arterioles by 9 ± 1% at 10<sup>-8</sup> and by 16 ± 1% at 10<sup>–6</sup> mol/l (n = 47, p < 0.05). Arteriolar responses to oxytocin did not change with repeated applications (n = 10). Following hypoxia, dilator effect of oxytocin was not changed at 10<sup>–8</sup> (8 ± 2%) but it was reduced at 10<sup>–6</sup> mol/l (7 ± 2%; p < 0.05, n = 8). After asphyxia or ischemia, oxytocin did not dilate arterioles at 10<sup>–8</sup> mol/l, whereas 10<sup>-6</sup> mol/l resulted in a mild vasoconstriction (-4 ± 3 to -6 ± 4%, n = 6 and 8). Topically applied superoxide dismutase did not preserve arteriolar responses to oxytocin after asphyxia although the arterioles did not constrict to 10<sup>–6</sup> mol/l oxytocin (n = 5). Dilatation of cerebral arterioles in response to oxytocin was reversed to constriction by N<sup>ω</sup>-nitro- L-arginine methyl ester ( L-NAME) (15 mg/kg, i.v.; n = 5) and by endothelial impairment by intra-arterial infusion of phorbol ester (10<sup>–5</sup> mol/l; n = 5). We conclude that the absence of pial arteriolar dilation to oxytocin after ischemia and asphyxia indicates endothelial dysfunction which may be involved in the pathology of perinatal brain injury.