A detailed analysis of the myometrium showed that the physical state of phospholipids--fluidity--depends on the ratio of their individual components and changes in relation to the state of contractile activity. The results indicate that before term of labor changes in the fluidity of myometrial phospholipids as well as increased availability of arachidonic acid for prostaglandin synthesis can induce preterm onset of contractile activity. Possibilities of affecting these mechanisms, which belong to the many potential factors inducing preterm labor, are discussed.