Progesterone is the natural progestagen produced by the corpus luteum during the luteal phase. It is absorbed when administered orally, but is greater than 90% metabolized during the first hepatic pass. This greatly limits the efficacy of once-daily administration and also results in unphysiologically high levels of progesterone metabolites, particularly those reduced at the 5-a position. These metabolites can cause dizziness and drowsiness to the point of preventing the operation of a motor vehicle. Synthetic progestins, such as medroxyprogesterone acetate and norethindrone acetate (NETA), have been specifically designed to resist enzymatic degradation and remain active after oral administration. However, these compounds exert undesirable effects on the liver and often cause severe psychological side effects. The permeability of the skin does not allow for administration of progesterone in the quantities normally produced by the corpus luteum, i.e., up to 25 mg/day during the mid-luteal phase. To avoid this problem, synthetic progestins such as NETA have been administered transdermally. These compounds, though, just like synthetic estrogens administered non-orally, retain undesirable hepatic effects even when administered transdermally. Transvaginal administration of progesterone is a practical non-oral route available for administering progesterone. Early experience was gained with vaginal suppositories, which lack manufacturing controls. Recently, a new progesterone gel formulation has been designed for vaginal use. The clinical acceptability of this product has been enhanced by the bioadhesive characteristics of its polycarbophil-based gel, which conveys controlled and sustained-released properties. Investigations have shown that because of local direct vagina-to-uterus transport, which results in a preferential uterine uptake of progesterone, this formulation given in conjunction with physiological amounts of estradiol produces endometrial changes similar to those seen in the luteal phase, despite plasma progesterone levels that remain subphysiologic. Studies in infertility show that vaginal progesterone in this form allows secretory transformation of the endometrium and the development of pregnancy despite providing low systemic progesterone concentrations. Fewer side effects occur when used for hormone replacement than typically encountered with progestins and oral progesterone. Uses in patients with infertility and hypoestrogenism and secondary amenorrhea are reviewed.