Tamoxifen is a nonsteroidal anti-estrogen which is prescribed for treatment of breast cancer. It is an estrogen receptor modulator. Dependent on the target tissue, It may be estrogen agonist or antagonist. Some effects of the reproductive tract have been observed. The purpose of this study was to investigate the tumorigenesis effects of neonatala dministration of tamoxifen on endometrium in female mice. In this study 10 females were examined. Female mice were divided into test and control groups. Control group received sesame oil and treatment groups received tamoxifen (400 Âμg/kg/days) per 1 to 5 days after birth subcutaneously. All animals were perfused with paraformaldehyde in 70 days and uterus were removed and post-fixed in paraformaldehyde and cross-section of uterus stained with hematoxyline and eosine and were evaluated histologically. In results, we found thikness in endometrium stroma. Morphological and histological studies showed that tamoxifen affects on endometrium stroma and causes of its thickness and decrease body weight (p ≤ o.oo1). Tamoxifen, a selective estrogen modulator (SERM) is an agonist in the uterus. These agonist actions in the uterus may cause an increased risk of endometrial cancer. Howsoever, many studies have shown that tamoxifen can act as an estrogen agonist in the female genital tract leading to the development of endometrial hyperplasia and sometimes malignancy. In many patients, tamoxifen has an antiestrogenic effect on the uterus, and patients can develop inactive or atrophic endometrium. Further studies are essential to investigate other effects of tamoxifen on the female uterus.

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