In order to clarify the biochemical bases of myometrial contractions, (Ca2+-Mg2+)-ATPase activity, actomyosin (ACT)-ATPase activity and ACT superprecipitation were studied during pregnancy. The results indicate that; (Ca2+-Mg2+)-ATPase activity in nonpregnant ovariectomized rats was markedly increased after estradiol treatment (15 micrograms/day for 3 days) from 12.0 +/- 20.1 to 132.5 +/- 22.8 nmolePi/mg/min (p less than 0.01). The activity on days 19, 20, 21 and 22 of pregnancy was 48.0 +/- 5.0, 48.2 +/- 7.5, 112.3 +/- 18.5 and 27.3 +/- 8.8 nmolePi/mg/min, respectively, showing marked prepartum increase and rapid decrease after delivery. In human myometrium, basal (Ca2+-Mg2+)-ATPase activity showed no significant change but at term the activity was decreased (79.4 +/- 9.8 to 60.7 +/- 7.4 nmolePi/mg/min, p less than 0.05) in the presence of calmodulin (CMD). Human myometrial ACT-ATPase activity, on the other hand, was stimulated with CMD (1st trimester, term without labor and with labor: 295 to 1,134, 550 to 1,243 and 897 to 4,735 pmolePi/mg/min, p less than 0.05). Myometrial CMD concentrations, however, showed no change during pregnancy. ACT prepared from rabbit uterus showed an enhanced interaction of contractile proteins in the presence of CMD. These data indicate that CMD stimulates (Ca2+-Mg2+)-ATPase activity in early pregnancy but inhibits at term and increases ACT-ATPase activity. Since the myometrial CMD concentration remains unchanged during pregnancy, there may exist a function which alters CMD action on ATPase activity as pregnancy advances.