OBJECTIVE: To determine the effects of the last hMG administration on plasma P and androgen profiles during controlled ovarian hyperstimulation (COH) for IVF-ET. DESIGN: Controlled clinical study. SETTING: The IVF-ET program of a tertiary outpatient care center, Hopital A. Beclere, Clamart, France. PATIENTS: Nine IVF-ET candidates aged 25 to 36 years having presented normal responses to COH in previous IVF-ET cycles. INTERVENTIONS: Controlled ovarian hyperstimulation was induced for IVF-ET using hMG after endogenous gonadotropins were suppressed with a time-release GnRH agonist. Just before the last hMG administration (225 IU), the participants were hospitalized for 24 hours for serial blood sampling. These occurred before (baseline) and after hMG administration, every 30 minutes for 1 hour, hourly for 4 hours, and every 3 hours for the remaining part of a 24-hour post-hMG observation period. MAIN OUTCOME MEASURE: Measurement of P, T, androstenedione (A), E2, FSH, and LH. RESULTS: Plasma P and androgens (T and A) increased significantly, reaching peak values 12 to 15 hours after hMG administration and decreased progressively thereafter, to reach values not significantly different from baseline 24 hours after hMG administration. Plasma E2 levels increased progressively and steadily during the 24-hour observation period. Plasma FSH levels remained constant after hMG administration while LH stayed undetectable. CONCLUSION: In COH cycles induced for IVF-ET, the hormonal profile after the last hMG injection suggests that hMG triggers an increase in plasma P and androgens that culminates 12 to 15 hours after hMG administration. This elevation in plasma P and androgens observed after hMG administration is likely to reflect a direct action of the LH and/or FSH components of hMG on granulosa cells. In some women these hormonal consequences of hMG treatment may impair endometrial receptivity in IVF-ET cycles.