Absorption of micronized 17β-estradiol (E2), after intravaginal administration of 1 mg dose, suspended in saline, is extremely rapid and sustained. A mean peak increment of circulating E2 concentrations of more than 110 times the basal level is achieved at 2 h and remains elevated more than 6 times the basal level even at 24 h. Increments in estrone (E1) are smaller and slower than those of E2 with a mean peak concentration of less than 10% of E2 and remain 3 times the basal level at 24 h. With 0.5 mg dose, the incremental changes in E2 and E1 as well as the degree of gonadotropin suppression are essentially the same. In contrast, intranasal administration of E2 of 1 mg dose induces a rapid but short-lasting increase in both serum E2 and E1 levels. The mean increments of E1 exhibited a higher and sustained elevation with a rise in mean E1/E2 ratio well above unity beginning 1 h after intranasal application. These findings indicate intravaginal but not intranasal routes of E2 absorption are quantitatively much greater and circumvent the local conversion of E2 to E1 observed after oral administration of E2, and thus represent a practical and highly effective means of delivering E2 into the circulation.