A 11.20-mg dose of progesterone was administered by nasal spray to five healthy fertile women in the follicular phase of the menstrual cycle. Serial blood samples were collected and Cmax (the maximum progesterone concentration reached), Tmax (the time at which Cmax was reached) and the area under the curve (AUC), with the time limits of 0 and 720 min, were calculated. Serum progesterone levels were assayed by means of a non-extraction [125I]radioimmunoassay. The mean Cmax was 4.50 +/- 2.31 ng/ml at a Tmax of 30 min; levels returned to baseline after 720 min. The mean AUC value was 1180.50 +/- 613.90 ng.h/ml. The progesterone administered by nasal spray in fertile women was effective in reaching physiological progesterone levels. Even if a nasal first-pass metabolic effect is taken into account, this route allows progesterone to avoid liver first-pass metabolism and its metabolic consequences.