The early effect of estrogen on the synthesis of cytosolic proteins was investigated in the luminal epithelium, endometrial stroma and myometrium of the uterus in adult ovariectomized rats. The procedure of Reiss and Kaye (1981) was followed (involving two-step fractionation of 35S-labelled proteins and fluorographic analysis) except that the uteri were fractionated into their three main tissue components before homogenization. The results show that E2 stimulates the synthesis of BB-CK (brain-type creatine kinase), the major component of IP (estrogen-induced protein), in the three tissues. This suggests that BB-CK is related to a function that is common to the estrogen responses (such as hypertrophy) of all three uterine tissues in ovariectomized adult animals. The synthesis of two unidentified proteins of 37000 and 27000 Mr was markedly stimulated in the epithelium. These proteins are probably rate-limiting in responses to estrogen treatment that are specific to the epithelium. The 27000 Mr protein has the same charge as that of the 27000 Mr nafoxidine-induced protein described previously (Mairesse et al., 1981) and is probably therefore the same protein.