During normal pregnancy, and especially in the third trimester, both uterine blood flow and prostacyclin production by ovine uterine arteries are dramatically increased. We sought to determine if this is due, in part, to an increase in cyclooxygenase (COX) expression in the uterine artery endothelium. In this study we compared COX expression in uterine artery endothelium from nonpregnant and third-trimester pregnant (110-142 days' gestation) ewes. COX-2 expression was not detectable by Western blotting in uterine artery endothelium or vascular smooth muscle (VSM). In contrast, COX-1 expression was clearly observed in uterine artery. Immunohistochemical localization of COX-1 was endothelium > VSM, with both cell types showing an increase in COX-1 during the third trimester of pregnancy. COX-1 protein and messenger RNA (mRNA) levels were also detectable in collagenase dispersed endothelial cells, with expression of COX-1 in uterine artery endothelial cells dramatically increased during the third trimester of pregnancy at both the level of protein (346.4 +/- 28% of nonpregnant controls, P < 0.0005) and mRNA (51.04 +/- 7.98-fold of nonpregnant controls, P < 0.001). We conclude that the pregnancy-induced increases in prostacyclin production by uterine arteries is largely due to a dramatic increase in expression of COX-1 mRNA and associated protein predominantly occurring in the uterine artery endothelium and, to a lesser extent, in the VSM.