Recently, 6% of 1,176 Sprague Dawley rats examined in our reproductive toxicity studies presented with dark-red uterine contents with or without fetuses demonstrating delayed development. Sometimes, a high proportion of the litter was found dead, and dystocia with death or preterminal euthanasia of the dam occurred. Microscopic findings in the uterus consisted of necrohemorrhagic and suppurative periplacentitis associated with the presence of bacterial colonies identified as Escherichia coli. In the vagina, similar findings were observed that were associated with mucus accumulation and the presence of a transverse occlusive or partially occlusive thin membrane identified as a vaginal septum. Microscopically, this septum consisted of a thin band of connective tissue covered on both sides by a mucous epithelium that was continuous with vaginal epithelium. In some cases, there was only mucus accumulation retained by a septum in the vagina without evidence of bacterial infection. Serological and histological examinations did not reveal any specific pathogenic agent. The presence of these septa in the vagina most likely favored mucus accumulation, nonspecific ascending bacterial infection, and dystocia. This colony of rats presented with an unusually high incidence of vaginal septa as it was described in different strains of mice and rats in the past. We hypothesized that the use of an impedance meter by the breeder--to determine the phase of the estrous cycle by introducing a probe in the vagina--likely facilitated gestation by perforating the vaginal septum in some cases.