In uteri from adult female rats, Thymidine Kinase activity varied during ovarian cycle and was maximum at metestrus. Subcutaneous injections of 5, 10 or 25 ng of estradiol-17 beta to immature female rats, resulted in a 3, 5 or 10 fold increase of enzyme activity. Moreover, Thymidine Kinase activity was decreased by injection of medroxyprogesterone alone or associated with estradiol-17 beta. From these results it seemed that Thymidine Kinase activity in rats uteri was specifically induced by estradiol-17 beta.