Introduction: Xenoestrogens are a group of environmental chemicals that imitates endogenous estrogens. Female reproductive system development and function during neonatal critical period may be susceptible to possible harmful effects of such environmental toxicants. In this study, it has been intended to determine the estrogenic potencies of chemicals, Zearalenone (an estrogenic mycotoxin which contaminate cereals) and Tamoxifen (an anti estrogen drug to reduce breast cancer risk in women). Methods: Neonatal female mice received subcutaneously injections of sesame oil, 20μg estradiol benzoate (EB), 0.5mg Zearalenone (ZEN 0.5), 1mg Zearalenone (ZEN 1), 2mg Zearalenone (ZEN 2), 400μg Tamoxifen (Tax), ZEN 0.5+ Tax, ZEN 1+ Tax and ZEN 2+ Tax per kg of body weight on postnatal days 1–5, and controls did not receive any treatment. Estrus cycle and lordosis behavior were monitored after vaginal opening to PND 70. Results: observations showed vaginal opening was significantly advanced by EB, ZEN 1, ZEN 2, Tax, ZEN 0.5+ Tax, ZEN 1+ Tax and ZEN 2+ Tax compared to control animals and was accompanied by lower numbers estrus cycle and lordosis behavior. Conclusion: neonatal exposure to ZEN and TAX accelerated the onset of puberty but decreased reproductive parameters in female mice. In addition, Tamoxifen act more like an agonist than antagonist on the reproductive functions.

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