Our objectives were to determine the rate and extent of follicular evacuation during human ovulation. Female volunteers were given 5000 or 10,000 IU human chorionic gonadotropin to induce ovulation when a follicle reached 18-20 mm. Ovulations were observed via transvaginal ultrasonography and recorded. Images were digitized for computer-assisted analysis. Areas of the follicles, taken to reflect follicular volume, were measured at specific time intervals from the moment of the first leakage of fluid until complete follicular evacuation. Twenty-five ovulations were visualized. In 23 cases (92%) all of the follicular fluid was expelled, but in two cases the follicle failed to empty completely. In all complete ovulations the initial fluid loss was rapid. The time to reach 70% evacuation was 0.9 +/- 0.3 min (+/- SEM). The remaining fluid leaked out more slowly; the mean time to complete follicular evacuation was 6.1 +/- 1.1 min. There was considerable variability in the rates of ovulation (range 6 s to 18.5 min). An association was observed between the type of stigma formed prior to ovulation and the initial rate of fluid loss. The data provide evidence of variability during follicular evacuation, which may in turn influence the successful expulsion of the oocyte from the follicle.