While the mechanism of the initiation of labor in humans has not been clarified satisfactorily, it is of major clinical interest, particularly with a view to understanding and avoiding preterm labor. Progesterone, whose role can now be determined in greater detail by the use of newly developed progesterone antagonists, and estrogens both play a role. Recently, attention has focused not only on contraction-stimulating substances such as oxytocin and prostaglandins, but also on cytokines, which have been implicated in the pathogenesis of preterm labor related to intrauterine infection. A model describing the various steps leading to regular uterine contractions is discussed and the resulting implications on stimulation and inhibition by pharmacological substances are outlined.