In vitro binding and exchange methods were used to determine the levels of oestradiol (E) and progesterone (P) receptors in cytosolic and nuclear fractions of cells obtained from the porcine myometrium and cervix during pregnancy and parturition. The concentration of oestradiol cytosolic receptors (ERc) in non-placental and placental myometrium and the cervix was low in early and mid-pregnancy, increasing at late pregnancy and was highest at parturition. The oestradiol nuclear receptors (ERn) followed the same pattern in the 3 sampling areas. The levels of PRc in non-placental regions were low in early pregnancy, increased to late pregnancy but declined during parturition. In placental regions the PRc value was high in early and mid-pregnancy, but decreased at late pregnancy and parturition. The concentration of PRc in the cervix increased significantly at mid-pregnancy and declined afterwards. In early and mid-pregnancy the PRn content was high, and decreased at late pregnancy and parturition in the 3 sampling areas. The data obtained agree with the present theories on the endocrine mechanisms regulating receptor levels in the uterus. Moreover, these data support the concept that the uterine motility during pregnancy and parturition might be influenced by changes in myometrial steroid receptor concentrations.