Estradiol-17β, diethylstilbestrol (DES) and certain phytoestrogens were administered intraperitoneally to immature female rats. Though coumestrol and genistein exhibited uterine responses similar to those of estradiol-17β and DES, the dose required to produce the effects was approximately 102–103 times higher than in the latter two compounds. In vitro incorporation of 3H-uridine in rat uterus showed that common feedstuffs containing phytoestrogenic activity influence biochemical processes preceding net increases in target tissue synthesis.