Electrophoretic systems were used to quantify and characterize the main proteins involved in myometrium contraction, i.e., myosin, actin, and tropomyosin. Skeletin, a cytoskeletal component was also investigated. No difference in their respective concentrations could be found in 13 uteri representing various menstrual cycle stages, postmenopausal conditions, and different gestational ages. The great variability of their tissular level observed in each of the uteri does, however, reflect the heterogeneity of the muscular component of the uterine wall. Comparison of the different forms of actin in gravid and non-gravid uteri showed an increase of the γ-form in gravid uteri. However, myosins purified from gravid and non-gravid uteri were shown to have identical ATPase activity and peptide patterns following limited chymotryptic proteolysis