These studies were undertaken to examine the interaction of tamoxifen with sex steroid hormones in rat uterine activity. The uterine wet weights of the immature Tat uterus were examined after the administration of estradiol-l7$eta$(1$mu$g), tamoxifen(50$mu$g), progesterone(lmg). The uterotropic activity in immature ovariectomized rats was observed under various treatment conditions following pretreatment with above drugs. The results obtained were as follows:1) Tamoxifen produced significant increase (p <0.01) in uterine wet weight compared with control group, although the increase was not as great as that seen with estradiol-17$eta$. Administration of estradiol-17$eta$ together with tamoxifen inhibited significantly the increase of uterine wet weight by estradiol-17$eta$ (p < 0.01). Coadministration of progresterone with tamoxifen partly blocked the increase of tamoxifen-induced uterine wet weights by progesterone. 2) Estradiol-17$eta$after the estradiol-17$eta$ pretreatment discontinued the declining uterine wet weights due to the absence of estrogen support, but uteri continued to increase in weight if daily estradiol-17 $eta$ was maintained. Administration of tamoxifen on the fourth day of estradiol-17$eta$ treatment reduced uterine wet weights within 24 hours, and the weights continued to decline with additional tamoxifen. 3) The modest growth of the uterus induced by three daily injections of 5Opg tamoxifen remained stable for five days, with or without additional tamoxifen treatment. Coadministration of tamoxifen with estradiol17$eta$ increased slightly the increase of uterine wet weight by tamoxifen. Coadministration of tamoxifen with progesterone inhibited the increase of uterine wet weight by tamoxifen. 4) The modest growth of the uterus induced by three daily injections of lmg progesterone reduced uterine wet weight to the control level for five days. Commencement of tamoxifen or estadiol-17 $eta$ injections on the fourth day of progesterone treatment rapidly elevated uterine wet weight.