ObjectiveTo elucidate the mechanism of cardiovascular protection of hormone replacement therapy (HRT) by comparing the effect of oral and transdermal HRTs on the production of antiaggregatory, vasodilatory prostacyclin, and its endogenous antagonist, thromboxane A2.MethodsOral estradiol (2.0 mg/d) plus norethisterone acetate (1.0 mg/d) (n= 13) or transdermal estradiol (50 μg/d) plus medroxyprogesterone acetate (10 mg/d) as 12-day courses at 4-week intervals (n= 13) were given to postmenopausal women. Urinary excretion of the metabolites of prostacyclin, ie, 6-ketoprostaglandinF1αand 2,3-dinor-6-ketoprostaglandinF1α, as well as those of thromboxane A2, ie, thromboxane B2and 2,3-dinor-thromboxane B2, were measured by radioimmunoassays, after purification by extraction and high performance liquid chromatography, before and during the sixth and the 12th treatment cycles.ResultsOral HRT stimulated excretion of thromboxane B2from 3.4 ± 0.7 ng/mmol creatinine to 4.5 ± 1.5 (mean ± standard deviation,P< .05) and that of 2,3-dinorthromboxane B2from 16.6 ± 8.0 ng/mmol creatinine to 26.2 ± 10.7 (P< .01), and thus led to the dominance of thromboxane A2. No changes in prostanoids occurred during transdermal HRT.ConclusionsThe effects of various HRTs on prostanoids may significantly differ.