Progesterone is known to be involved in many steps in female reproduction including control of implantation and uterine-cervical function during pregnancy. Our studies in rats and guinea pigs indicate that progesterone inhibits uterine contractility and cervical softening during pregnancy. Progesterone levels or actions decline near the end of pregnancy leading to the onset of labor. Treatment with progestin agonists prolongs pregnancy and inhibits cervical softening, whereas treatment with antiprogestins (mifepristone or onapristone) stimulates uterine contractility, cervical softening and premature delivery. Thus the effect of progesterone receptor modulators in the uterus and cervix depend up on the degree of intrinsic agonistic/antagonistic activities. Our recent studies show that progesterone interacts with nitric oxide (NO) to maintain pregnancy and that administration of progesterone antagonists with NO synthase inhibitors act synergistically to stimulate labor. In addition our studies show that combinations of progesterone antagonists with aromatase inhibitors act synergistically to induce labor. Similarly antiprogestins interact with NO synthase or aromatase inhibitors to block implantation through action on the endometrium. These studies suggest new applications for combined therapies of progestin receptor modulators with aromatase inhibitors or agents that modify NO production for contraception, stimulation of labor, estrogen-dependent diseases and improved outcomes in pregnancy.