Forty-four infertile women in 44 cycles were stimulated by CC/hMG/hCG (39 cycles) and by CC/hCG (5 cycles) for the purpose of in vitro fertilization (IVF, 37 cases with 35 transfers) and gamete intrafallopian transfer (GIFT, 7 cases). Intraoperative ovum pick-up was performed 32-36 hours after hCG injection. Blood E2 and P levels were determined. Blood E2 was found to be proportional to the number of mature and intermediate oocytes obtained, and correlated with the number of follicles (greater than 10 mm) and the total follicular fluid volume. Altogether 213 follicles aspirated were analyzed. There was a highly significant linear correlation (r = 0.72, P less than 0.01) between the follicular diameter (X cm) on the operation day and the aspirated follicular fluid volume (FFV, Y). The equation of the line was Y = 3.8 X-4.4. The higher concentrations of follicular E2, P, FSH, LH and lower T concentration were crucial to the maturing process of oocytes. It was found that mature oocytes could be retrieved from 26.5% of the small follicles, so it is worthwhile to aspirate the small ones. The fertilization rate was much higher with the mature oocytes than the immature ones. But the morphologically mature oocytes might function differently, i.e. the mature oocytes retrieved from the small follicles or from follicles with unbalanced hormonal microenvironment usually give a low fertilization rate.