Subcutaneous injection of 50 micrograms of a luteinizing hormone-releasing factor agonist (LRF agonist) for three successive days at the time of menstruation in normal cycling women induces a shortened luteal phase with suboptimal concentrations of circulating estradiol and progesterone. This luteal phase defect follows a reduced concentration of follicle-stimulating hormone during the follicular phase and a resulting inadequate follicular maturation. Since a short luteal phase is associated with an endometrium not conductive to implantation, administration of the LRF agonist at the onset of menstrual cycle may prove to be a practical and novel approach to fertility control.