Oral estrogen treatment increases thrombotic risk. Tissue factor (TF), tissue factor pathway inhibitor (TFPI) and platelet interaction with leukocytes are important determinant of thrombogenesis. Thereby, the present study was designed to define and compare platelet TF and TFPI mRNA and adhesion protein expression in platelets derived from animals treated with different types of oral estrogens. Ovariectomized pigs were treated with either 17 -estradiol (2mg/day), conjugated equine estrogen (CEE) 0.625mg/day, or raloxifene 60mg/day for four weeks. Compared to intact animals, ovariectomy and treatment differentially affected populations of leukocytes: neutrophils decreased whereas lymphocytes increased significantly four weeks after ovariectomy and with 17-estradiol and CEE treatments; eosinophils increased only with 17-estradiol treatment. Content of TF protein increased in platelets from 17-estradiol and raloxifene treated pigs whereas TF mRNA was detected only in platelets from 17-estradiol and CEE treated pigs. TFPI mRNA increased in platelets following OVX and estrogen treatment. Only a trace of TFPI protein was detected but a higher molecular weight protein was observed in all treatment groups. Expression of CD40 and CD40 ligand increased with OVX and decreased with 17-estradiol and CEE treatments more than with raloxifene. The ratio of activated to basal P-selection expression decreased with ovariectomy and increased with raloxifene treatments. These results suggest that estrogenic formulations may affect individual thrombotic risk by different mechanisms which regulate TF and platelet-leukocytic interactions. These studies provide the rationale for evaluation of interactions among platelets, TF and TFPI expression on thrombin generation during estrogen treatment in humans.