Growth, reproduction, circulation, and thermoregulation in domestic livestock and laboratory rodents are adversely affected by consumption of tissues derived from plants infected with endophytic fungi. Because little is known about the systemic effects of infected diets on wild rodent granivores, we conducted a series of laboratory experiments to assess the effects of consuming endophyte-infected (E ) and uninfected (E−) seeds of tall fescue (Festuca arundinacea) on reproduction in the white-footed mouse, Peromyscus leucopus. Groups of mice (paired and single-sex) were fed diets of either rodent chow, E , or E− seeds; in several experiments, chow and seeds were ground and mixed in a 1:1 ratio. E− seeds alone suppressed reproductive output compared to chow-fed mice, but consumption of E seeds did not further reduce reproductive performance. Furthermore, paired testes mass was more strongly reduced by the presence of seeds in the diet than was the mass of the female reproductive tract, but reproductive tract mass in both sexes was not further diminished by endophyte consumption. These results suggest that males are more sensitive than females to the presence of seed in the diet, and that white-footed mice and related rodent granivores cannot rely upon a diet of grass seeds alone to support reproduction. Depending upon the time of year that such seeds are eaten, suppression of breeding may be advantageous to consumers.