OBJECTIVE: To verify the occurrence of preferential distribution of vaginally administered progesterone to the uterus compared with extrapelvic regions in vivo and in humans. DESIGN: Prospective clinical study. SETTING: University medical school. PATIENT(S): Twenty postmenopausal women undergoing transabdominal hysterectomy for benign pathologies. INTERVENTION(S): Forty-five minutes before surgery, the women received a single vaginal administration of an oil-based micronized progesterone (100 mg) solution currently available on the market for IM use. During the operation, parallel blood samples were drawn from the uterine and radial arteries. MAIN OUTCOME MEASURE(S): Plasma levels of progesterone were measured by RIA. RESULT(S): Mean (+/- SD) plasma levels of progesterone were significantly higher in the uterine artery than in the radial artery (9.75 +/- 3.21 vs. 5.12 +/- 2.06 ng/mL, respectively). CONCLUSION(S): Vaginal administration allows a preferential distribution of progesterone to the uterus, which confirms the existence of the so-called "first uterine pass effect."