Ovulation was induced in 24 cycles of normal ovulatory patients for in vitro fertilization using a modified human menopausal gonadotropin and human chorionic gonadotropin (hMG/hCG) protocol. This modification was based on experience in 31 cycles previously studied. The individual ovarian threshold response to hMG seen previously was again confirmed, and successful stimulation and oocyte retrieval depended upon the recognition of this patient's "sensitivity." A rapid serum estradiol (E2) assay, in conjunction with estimation of the patient's biologic response to the E2 levels, as measured by changes in vaginal smears and cervical mucus (biologic estrogen shift), was the key to determining the amount of hMG necessary. The biologic shift was the best indicator for discontinuation of gonadotropins in the low E2 responder group. The serum E2 levels were the best indicators for the normal and high E2 responder groups. Ultrasound was used as confirmatory evidence for satisfactory follicular development. Postmaturity of the oocyte did not occur in this series, due to compensation of excessive hMG stimulation in rapid responders by a shortening of the interval between hMG discontinuation and hCG administration, thus initiating the oocyte meiotic process earlier. Maturation of immature oocytes was accomplished in this series by an increase in the time in culture prior to insemination. Due to the improved protocol of drug administration and the ability to mature immature oocytes in vitro, five pregnancies resulted from the stimulation of these 24 cycles.