Correct timing of human chorionic gonadotropin (hCG) administration in induced cycles for in vitro fertilization is of crucial importance to oocyte maturation and normal luteal function. The purpose of this work was to compare the effect of hCG timing on follicular development, oocyte maturation, and fertilization in vitro, as well as on the pattern of luteal phase hormone secretion. Ovulation was induced in 32 normally cycling women by human menopausal gonadotropin (hMG)/hCG administration. In the first group (17 women) 10,000 IU hCG was administered 24 hours after the last injection of hMG and in the second group (15 women) 48 to 72 hours after the last hMG injection. Serum estradiol levels prior to oocyte aspiration were similar in both groups, as were the numbers of large follicles on the day of hCG administration (4.5 +/- 2.3 versus 4.1 +/- 1.9 follicles/woman, respectively). The distribution of oocyte-corona-cumulus complexes was similar in both groups and was comprised of 11% immature, 43% intermediate, and 45% mature complexes. The fertilization rate, however, was significantly (P less than 0.001) reduced in the group treated by delayed hCG injection (57% versus 84%), and the percentage of degenerated oocytes was increased (9% versus 1%). Luteal phase length as well as progesterone and estradiol levels were comparable in both groups. It is concluded that an interval longer than 24 hours between the last injection of hMG and the administration of an ovulatory dose of hCG does not affect follicular and luteal phase serum steroid patterns but may result in a decreased oocyte fertilization rate, possibly due to atretic changes in the follicles.