We have shown previously that estradiol, estriol, and Nafoxidine (Upjohn 11, 100A) have differential effects on uterine growth and that these effects are associated with the retention of the estrogen receptor by the nucleus. In order to examine these relationships further, we have studied the effect of these hormones on endogenous nuclear RNA polymerase I and II in the immature rat uterus. All three compounds caused a rapid elevation in polymerase II activity that reached a peak by the first hour and declined to almost control levels by 2 h after the injection. This transient peak in polymerase II activity was followed by a second elevation by the fourth hour in estradiol- and Nafoxidine-treated animals which was not observed in estriol-treated rats. The activity of polymerase I increased monotonically to very high levels by 4 h and was maintained 12 h or longer in estradiol- and Nafoxidine-treated animals. A similar elevation was observed in estriol-treated rats but the activity declined very rapidly to control levels by 12 h. The second elevation in polymerase II activity and the sustained stimulation of polymerase I activity were correlated with the stimulation of true uterine growth. These data confirm our previous suggestion that long-term nuclear retention of the receptor is a requirement for true uterine growth and suggest that an obligatory response in the production of true growth is the stimulation of a second rise in polymerase II activity and an elevated and sustained activity of polymerase I.