During the third year of the project in vitro observations of isometric contractions of uterine muscle of ovariectomized mature rats will be employed in a study of the effects of various combinations of estrogens and progestins on contractility patterns of uterine muscle in this species. Uterine muscle contractions will be observed in lightly anesthetized mature rats by means of miniature transducers in (1) normal animals at the time of proestrus and estrus; (2) in ovariectomized rats which have received no hormonal treatment, and, (3) in ovariectomized rats which have received injections of estrogens and progestins as previously described for in vitro observations.