A progesterone solution was administered by intravaginal instillation, intramuscular injection, and sublingually to estrogen-deficient women, with or without estradiol (E2) replacement, and serum progesterone (P) concentrations were measured by radioimmunoassay. Intravaginal application to postmenopausal subjects receiving E2 gave the highest values of serum P: 10 times baseline at 15 minutes and 30 to 40 times at 1 to 2 hours, with sustained levels, for 7 hours and decline to 10 times baseline at 24 hours. Intravaginal application to hypoestrogenic women gave similar results, but of much lower magnitude (highest value, 20 times baseline). Intramuscular injection, in contradistinction, showed gradually increasing levels over the study period, up to 30 times basal values at 24 hours. It contrast, sublingual application produced very modest serum increases, to approximately 10 times baseline within 2 hours and return to basal value at 24 hours. Since the most rapid and highest levels were observed by vaginal application to postmenopausal women receiving estrogen, and considering that the vagina has been similarly shown to be very effective for estrogen absorption, it is conceived that full hormone replacement could be accomplished in the deficient states by cyclic vaginal application of both steroids.