This article presents a review of studies concerned with the mechanism by which estrogens stimulate RNA and protein synthesis in target organs. Two model systems are discussed in detail: the rat uterus and the chick oviduct. It appears that the sequence of events is similar in both organs. Based on available evidence, estrogen is bound to a cytoplasmic receptor protein immediately upon entering the cell. This complex subsequently moves into the nucleus where it apparently interacts with specific nonhistone proteins of the chromatin material. This initial interaction results in synthesis of new RNA molecules. These new RNA's are evidently required in the cytoplasm to allow the synthesis of a select complement of new proteins. Some of these proteins may feed back upon the nucleus thus activating the general gene amplification response that results in the overall growth and differentiation of the target organ.