The objective of this study was to assess the effects of endophyte-infected tall fescue (G1-307) seed (80% infected, by Acremonium coenophialum) on the estrous cycle in CD-1 mice. Twelve 60-day-old CD-1 female mice were randomly allocated to one of two dietary treatments containing 50% laboratory chow and (1) 50% endophyte-free (noninfected) fescue seed; (2) 50% endophyte-infected fescue seed. All mice were fed ad libitum. On day 35 of the study all mice were allowed access to a diet containing 100% laboratory chow for 7 days. Vaginal smears of each female were prepared daily throughout the 42 day trial period. All vaginal smears were stained and evaluated to determine phase of the estrous cycle. The days (+/- SE) to the first abnormal occurrence in the estrous cycle for treatments one and two were 27.0 +/- 5.6 and 16.7 +/- 5.2, respectively. The percent abnormal cycles of the total cycles observed between treatments one and two were 20.5 +/- 6.1 and 39.3 +/- 12.5, respectively. Mice on treatment two exhibited a greater frequency of abnormal sequences of the phases of the estrous cycle and greater cycle length (P less than 0.05). The results suggest that the infected fescue seed had a significant effect on the estrous cycle of the mouse (P less than 0.05).