Estrogen potentiates vascular reactivity to vasopressin (VP) by enhancing constrictor prostanoid function. To determine the cellular and molecular mechanisms, the effects of estrogen on arachidonic acid (AA) metabolism and on the expression of constrictor prostanoid pathway enzymes and endoperoxide/thromboxane receptor (TP) were determined in the female rat aorta. The release of TxA2 and prostacyclin (PGI2) were measured in male (M), intact female (InT-F), ovariectomized female (OvX-F) and OvX+17- estradiol-replaced female (OvX+ER-F) rats. Expression of mRNA for COX-1, COX-2, thromboxane synthase (TxS) and TP by aortic endothelium (Endo) and vascular smooth muscle (VSM) of these four experimental groups was measured by RT-PCR. Expression of COX-1, COX-2, and TxS proteins by Endo and VSM was also estimated by immunohistochemistry (IHC). Basal release of TxA₂ and PGI₂ was similar in M (18.8 ±1.9, 1,723±153 pg/mg ring wt./45 min., respectively) and InT-F (20.2±4.2, 1,488±123 pg, respectively). VP stimulated dose-dependent release of TxA2 and PGI2 from both M and F. OvX markedly attenuated and ER therapy restored VP-stimulated release of TxA2 and PGI2 in F rats. COX-1 mRNA levels did not differ in either Endo or VSM (P > 0.1). Expression of both COX-2 and TxS mRNA were higher (P < 0.05) in both Endo and VSM of InT-F and OvX+ER-F, compared to M or OvX-F. Expression of TP mRNA was significantly higher in VSM of InT-F and OvX+ER-F, compared to M or OvX-F. IHC revealed uniform staining of COX-1 in VSM of the four groups, whereas staining of COX-2 and TxS was greater in Endo and VSM of InT-F and OvX+ER-F than in OvX-F or M. Thus, estrogen enhances constrictor prostanoid function in F rat aorta by upregulating expression of COX-2 and TxS in both Endo and VSM and expression of TP in VSM.