We investigated if access to a running wheel after in utero exposure to benzyl butyl phthalate (BBP) ameliorates the toxicological effects of BBP exposure. Our purpose was to determine if post-birth exercise after prenatal BBP exposure could reverse alterations in mammary gland development. Twenty-five female pups were exposed to 500 mg/kg of BBP on days 9-15 in utero and analyzed for mammary gland development and morphology. Mice either had access to a running wheel beginning at 8 weeks of age or were in a cage with a "locked" wheel to prevent running activity. Whole mount staining showed delayed mammary gland morphology development, regardless of wheel exposure in the treated groups. Additional histology staining revealed BBP exposed mice that were not allowed exercise, had larger ducts with multiple cell layers containing proliferative cells suggesting a favorable environment for tumor growth. In addition, there was a significant increase in progesterone status in the mouse mammary gland at 20 weeks but not 10 weeks, regardless of wheel exposure. BBP exposure led to abnormal mammary gland development in female mice and access to a running wheel helped ameliorate some, but not all, of the harmful effects due to BBP exposure at either 10 weeks or 20 weeks of age. Our results are significant because they indicate that exercise can reverse most of the BBP-initiated alterations in the mouse mammary gland, and physical activity has a positive impact on most developmental parameters in the mouse mammary gland.