Actively growing culture of contagious equine metritis (CEM) bacteria or infective exudate (or both) were inoculated intrauterine in pony mares. A direct relationship was observed between (i) appearance and duration of cervicitis and vaginitis and (ii) vaginal exudate. Clinical signs appeared 1 to 3 days after mares were inoculated and lasted 7 to 23 days. In the acute phase of infection, all uterine and cervical samples yielded CEM bacteria. In the asymptomatic stage of infection, CEM bacteria were not isolated from uterine and cervical samples; however, in 33%, 28%, and 20% of the pony mares, CEM bacteria were present in clitoral fossa, clitoral sinus, and urethral vestibule, respectively, Sampling during early estrus increased the bacterial isolation rate to 57% in mares that were previously negative; however, 3 days later, CEM bacteria could not be isolated from 62% of the positive mares. The results of repeated exposure experiments indicated the presence of local antibodies, as no CEM bacteria could be recovered at 2, 7, and 15 days after reexposure with a small number of bacterial cells (8.4 x 10(5) cells). The CEM bacteria were isolated from all mares reexposed with a large number of bacterial cells (7.2 x 10(8)) at 2 days after second inoculation and from 50% at 7 days. However, all of the mares were negative by day 15 after reexposure, indicating increased resistance to CEM bacteria.