Introduction: Exposure to estrogen like chemicals during critical periods of development particularly in the neonatal period can disrupt normal patterns of mammary tissue development later in life. In this study, the effects of neonatal exposure of tamoxifen anti-estrogen and zearalenone mycoestrogen on some histological characteristics of mammary gland in female mouse were investigated.
 
Materials & Methods: Twenty newborn female mice of BALB/c strain were divided into four groups: three experimental and one control group (with no treatment). Treatment groups included Tamoxifen (400 μg /kg /day) group, Zearalenone treatment group (2 mg /kg /day) and group treated with tamoxifen+zearalenone treatment group. Daily treatments were started on the first day of life and continued up to 5 subcutaneously. Then about 70 days after birth, mice were deeply anesthetized and the mammary tissue was removed from the body. Following histochemical staining, the samples were studied using light and transmission electron microcopies.
 
Findings:  The result showed that epithelial thickness of milk duct and volume of the nipple in tamoxifen treatment group significantly (p<0.05) reduced. In contrast, the thickness of the epithelium and the nipple volume in tamoxifen+zearalenone and zearalenone treatment groups showed no significant difference in comparison with control. The electron microscopy of nipple duct showed negative effects of Tamoxifen on epithelium of milk duct.  
 
Discusion & Conclusions: The results of this study show that neonatal administration of tamoxifen probably exerts anti-estrogenic effects and it prevents the normal development of mammary tissue, but zearalenone do not have adverse consequences on mammary tissue.