Natural and synthetic progesterone have been used to treat luteal insufficiency, premenstrual syndrome, and in infertile patients. The transvaginal route has advantages, such as lack of local pain, avoidance of first-pass hepatic metabolism, rapid absorption, high bioavailability and local endometrial effect. The aim of this study was to evaluate the pharmacokinetic of natural progesterone administered as vaginal suppositories of 25, 50 or 100 mg. Thirty-five healthy ovulating patients, 31.54 ± 1.29 (mean ± SEM) years old, in the follicular phase of the menstrual cycle (between days 7 and 10) participated in the study. They were separated in three groups and received vaginal suppositories containing either 25, 50 or 100 mg of natural progesterone. Progesterone serum concentration reached maximal levels within 2 or 3 h after the administration and was similar for the three groups (7.27 ± 2.8 ng/ml; 8.84 ± 3.14 ng/ml; 9.82 ± 9.8 ng/ml, respectively). This study demonstrated that vaginally administered progesterone could reach levels that are similar to those obtained in ovulatory and luteal phases. The progesterone regimen for adequate endometrial protection and in vitro fertilization (IVF) programs still remains to be studied.