Different studies have focused on the effects of phytoestrogens-supplemented diets on mammary gland morphogenesis and breast cancer risk; however, particular dieting behaviors and food choices may result in a reduction of the natural source of phytoestrogens. The evaluation of a reduced phytoestrogens intake effect by depletion without modifying other dietary ingredients is hard. Since lignans, the largest contributors to phytoestrogens intake in Western diets, are metabolized into bioactive compounds by gut bacteria, long-term antibiotic treatments, inducing intestinal microflora disruption, may reduce enterolactone availability. To elucidate the effect of phytoestrogens lack on mammary tissue morphogenesis, female FVB mice were treated with gentamicin or metronidazole/ciprofloxacin from the age of 6 to 7 wk. After 21 wk, enterolactone urine levels were 120.07 +/- 20.5 ng/ml in untreated mice, 30.4 +/- 24.46 ng/ml in metronidazole/ciprofloxacin-treated mice, and 3.29 +/- 4.38 ng/ml in gentamicin-treated mice. Histological analysis revealed no significant alterations of mammary morphology in metronidazole/ciprofloxacin-treated mice, whereas gentamicin-treated mice showed increase of ducts number and duct-tree branching vs. controls. These findings indicate that normal mammary tissue size and shape are maintained even in the presence of low levels of lignans and suggest that only a complete depletion of these compounds induced significant alterations of mammary gland structure.