The purpose of the present study was to examine plasma progesterone (P4) changes around the time of the onset of the LH surge in women superovulated with clomiphene citrate and human Menopausal Gonadotrophin for in vitro fertilization (IVF). It was considered that changes in P4 levels which should be closely associated with the onset of the LH surge may be an accurate indicator for the time of oocyte recovery for IVF and that premature P4 secretion may prevent the establishment of pregnancy after embryo replacement. The plasma P4 concentrations determined at 0800, 1400, and 2100 h during the period 36 h before to 16 h after the onset of the LH surge in 72 women showed a significant diurnal variation with the nadir at 0800 h. The onset of the LH surge was detected in 51%, 32%, and 17% of patients at 0800, 1400, and 2100 h, respectively. The first significant increase in mean P4 concentrations was coincident with the onset of the LH surge at 1400 and 2100 h but because of the diurnal nadir of P4 at 0800 h, the increase in mean P4 levels was delayed until 1400 h when the LH surge began at 0800 h. Elevation of P4 concentrations from 2 to 8 nmol/liter in individual patients at 1400 and 2100 h before the onset of the LH surge did not prevent the establishment of pregnancy after embryo transfer. P4 concentrations before and after the onset of the LH surge were higher with increasing numbers of mature follicles. We conclude that changes in plasma P4 concentrations may be used to determine the time of ovulation or oocyte recovery for IVF because of their close association with the timing of the onset of the LH surge.