Chronic hypoxia during pregnancy has profound effects on uterine artery contractility and attenuates uterine blood flow. The present study tested the hypothesis that chronic hypoxia inhibits the pregnancy-induced reduction in pressure-dependent myogenic tone of resistance-sized uterine arteries. Uterine arteries were isolated from nonpregnant (NPUA) and near-term pregnant (PUA) ewes of normoxic control or high altitude (3,801 m) hypoxic (PaO2: 60 mmHg) treatment for 110 days. In normoxic animals, pressure-dependent myogenic response was significantly attenuated in PUA, as compared with NPUA. Hypoxia significantly increased myogenic tone in PUA, and abolished its difference between PUA and NPUA. Consistently, there was a significant increase in PKC-mediated baseline Ca²⁺ sensitivity of PUA in hypoxic animals. Hypoxia significantly increased PDBu-induced contractions in PUA but not in NPUA. Whereas the inhibition of ERK1/2 by PD98059 potentiated PDBu-mediated contractions of PUA in normoxic animals, it failed to do so in hypoxic animals. PDBu induced membrane translocation of PKC and PKC. While there were no significant differences in PKC translocation among all groups, the translocation of PKC was significantly enhanced in NPUA as compared with PUA in normoxic animals, and hypoxia significantly increased PKC translocation in PUA. In the presence of PD98059, there were no significant differences in PDBu-induced PKC translocation among all groups. Treatment of PUA isolated from normoxic animals with 10.5% O2 for 48 h ex vivo significantly increased PDBu-induced contractions, and eliminated its difference between PUA and NPUA. The results suggest that hypoxia up-regulates pressure-dependent myogenic tone through its direct effect in suppressing the ERK1/2 activity and increasing the PKC signal pathway, leading to an increase in the Ca²⁺ sensitivity of myogenic mechanism in the uterine artery during pregnancy.