Total rat uterine mRNA was isolated following 1,2 or 3 days of estradiol, 3 days of progesterone, or 3 days of progesterone and estradiol treatment of immature animals. The mRNA translation products were analyzed by 2-dimensional electrophoresis. The results demonstrate that a number of mRNA products were affected by varying the hormonal conditions. Estradiol consistently caused an increase in mRNA in vitro translation products corresponding to polypeptides with mol. wt of 40,000 Daltons (p40), 59,000 Daltons (p59). A polypeptide with a mol. wt of 37,000 Daltons (p37) consistently decreases in concentration following estradiol administration. Progesterone, alone, caused a decrease in p37, p59, and a slight increase in p40. When progesterone was combined with estradiol we detected translation product levels similar to those seen following 1 day of estradiol treatment. Thus, the change in mRNA populations as a response of the immature rat uterus to estradiol and progesterone can be utilized to characterize the tissue's response to these hormones. This type of study also provides a method for detecting gene products which may possibly be utilized as potential markers to investigate estrogen and progesterone action.