A breeder diet that shortens estrous cycles in mice has been reported to contain estrogenic substances, based on its ability to increase uterine weight of immature mice. However, the estrogenicity of the diet was inferred from uterine weight gain of immature mice that were intact. The increased uterine weight of mice on the breeder diet could thus have resulted from a precocious pubertal increase of endogenous estrogens induced by the diet rather than estrogenic substances in the diet. We therefore measured the estrogenicity of the breeder diet in ovariectomized animals. C57BL/6J mice were fed the breeder diet or a standard diet for 1 or 4 weeks. The breeder diet failed to increase uterine weights above control values for either treatment interval. Intact mice that were fed the breeder diet had twice the number of cycles of mice fed the standard diet, a confirmation of earlier studies. These results indicate that the breeder diet does not contain biologically significant estrogenic activity, and thus potentiates cyclicity by other means.