PIP: The combination of steroid hormones in oral contraceptives (OCs) acts both centrally and peripherally to alter normal reproductive function. Through the suppression of gonadotropin release, ovulation is inhibited. Follicular maturation is impaired as well. Even if ovulation occurs occasionally, the changes which occur secondary to chronic exposure to the progestin portion of OCs make both fertilization and implantation unlikely. The OC mechanism of contraceptive action on the central nervous system, on the hypothalamus and pituitary, and on peripheral reproductive organs is discussed. Multiple complex reactions occur in the hypothalamus and pituitary as a result of steroid ingestion. The clinical literature now agrees that the inhibition of pituitary gonadotropin secretion is the most important mode of action of OCs. Hormonal contraceptive preparations also affect the ovary, cervical mucus, the oviduct, and the endometrium. Sperm transport into the cervix is impeded by progesterone counteraction of estrogenic stimulation of cervical mucus penetrability. Through continued OC use, the endometrium becomes a hostile environment to implantation and further embryonic growth--due to glandular atrophy and stromal decidualization.