OBJECTIVE: To compare progesterone concentrations in serum and endometrial tissue from hysterectomy specimens after vaginal or intramuscular (IM) administration of progesterone gel. METHODS: This was a randomized open study of 14 post-menopausal women undergoing transabdominal hysterectomies. Participants received either vaginal progesterone gel, 90 mg, or IM progesterone, 50 mg, at 8:00 AM and 8:00 PM on the day before surgery and at 6:00 AM on the day of surgery. Venous blood samples for progesterone measurement were collected at 8:00 AM on the day before surgery (baseline) and during surgery. After removal of the uterus, the endometrium was sampled from the anterior and posterior walls. Results were expressed as ratios of endometrial to serum progesterone concentrations x 100. RESULTS: Ratios of endometrial to serum progesterone concentrations were markedly higher in women who received vaginal progesterone (14.1 median, 8.5-59.4 range; 95% confidence interval [CI] 9.89, 38.79) compared with IM injections (1.2 median, 0.5-13.1 range; 95% CI -0.48, 7.39) (P < .005). CONCLUSION: Ratios of endometrial to serum progesterone concentrations were higher after vaginal administration of progesterone than after IM injections. Our findings in endometrial tissue specimens from hysterectomies excluded the possibility of contamination by progesterone that remained in the vagina.