Platelet-activating factor (PAF) has been implicated in a number of reproductive processes ranging from ovulation to parturition. To examine the role of PAF in the human periovulatory processes, the PAF-acetylhydrolase (PAF-AH) activity was assayed in the follicular fluid (FF) obtained in conjunction with the in vitro fertilization and embryo transfer (IVF-ET) procedure and the activity related to oocyte maturation. The PAF-AH activity was also related to the concentrations of estradiol (E2) and progesterone (P) in FF. PAF-AH activity was significantly lower in the FFs obtained from follicles of more than 20 mm in diameter. The enzyme activity was significantly lower in the FFs of patients with a successful outcome of their pregnancies. E2 concentrations were negatively correlated with PAF-AH activities in the FFs. No correlation was found between the PAF-AH activity and concentration of P in the FF. Significantly more mature oocytes were recovered in the group who subsequently become pregnant compared to the non-pregnant group. It is suggested that PAF may be increased following follicular maturation. The increase in PAF may contribute to oocyte maturation and to the successful outcome of pregnancy following fertilization. An additional function of the increased PAF in FF may also be the stimulation of the contraction of smooth muscle in the ovary, thereby assisting the extrusion of the oocyte cumulus cell mass and signaling the completion of ovulation.