Observations on the steroid and peptide concentrations and interactions in the microenvironment of the human ovarian follicle have led to an understanding of three important issues in reproductive biology: (1) how the oocyte-granulosa-cell complex matures, (2) how a single follicle is selected and prepared for ovulation and (3) how the events are synchronized so that an ovum ready for fertilization is extruded at ovulation. These findings have clinical significance.