The effect of infection with Chalmydia trachomatis (mouse pneumonitis biovar) on the fertility of female mice was examined. Mice were inoculated into one ovarian bursa with approximately 10(4) inclusion-forming units of C. trachomatis and were mated five to 16 days later. At sacrifice all mice showed salpingitis or hydrosalpinx in the inoculated oviduct and implantations were found only in the uterine horn contiguous with the noninoculated oviduct. Animals inoculated with tissue culture supernate showed no tubal pathology and had implantations evenly distributed in both uterine horns. Intrauterine inoculation of Chlamydia five to 36 days before mating also resulted in tubal pathology and infertility. Intrauterine inoculation of C. trachomatis one to two days after mating (one to two days before expected nidation) had no effect on the number or distribution of implantation sites as compared with controls. These results demonstrate that chlamydial infection of the upper reproductive tract of mice prior to mating can result in infertility.