The uterus is an organ with an impressive capability to grow. It undergoes regular periods of growth during the normal female lifespan, i.e. during puberty, menstrual cycle/estrous and pregnancy. The exact mechanisms for the regulation of its growth is not known. Gonadal sex steroids (estrogens, progestins and androgens) are important regulators of several cellular functions in the uterus, including growth and differentiation. The effects of steroid hormones arise mainly from interactions with specific receptor-proteins in the target tissues and subsequent stimulation of gene transcription. The influence of estrogens on the proliferation of uterine cells is indirect, mediated via so called estromedins. Estrogens control uterine growth by regulating the synthesis of growth factors (e.g. IGF-I and EGF), their receptors, as well as other growth promoting substances (e.g. c-fos, c-jun and thioredoxin). In the uteri of ovariectomized (ovx) rats estradiol and androgens increase the mRNA levels of IGF-I and thioredoxin. Antiestrogens completely block the effect of estrogens on uterine weight and increased IGF-I and thioredoxin expression. The synthetic glucocorticoid dexamethasone attenuates the estrogen-induced increase of uterine IGF-I mRNA, but has not effect on thioredoxin mRNA. IGF-I given to ovx rats has estrogen-like effects. Progesterone alone does not affect the mRNA levels of IGF-I or thioredoxin in the uteri of ovx rats. In the endometrium of pigs during early pregnancy a peak in the IGF-I mRNA level is found concomitantly with a burst of estrogen from the embryos. This effect is not seen in the myometrium. In the human cervix the IGF-I mRNA level is increased in term pregnancy as compared to non-pregnant women. The level decreased again just after parturition. The thioredoxin mRNA levels are increased in the cervix of term pregnant and postpartal women as compared with the non-pregnant. In non-pregnant women the cervical thioredoxin mRNA level is positively correlated to the serum estradiol concentration. In this review the results from this lab will be discussed together with results from other groups on IGF-I and thioredoxin in uterine tissues, their regulation and relation to growth.