Contagious equine metritis 1977 was reproduced in pony mares using cultures of the Gram-negative coccobacillus aetiologically associated with the disease. Variability in clinical response was observed in the first of 2 experiments, with the presence of semen, either alone or in an extender, appearing to potentiate the pathogenicity of the challenge strain of the organism. The experimental disease was characterised by a variable degree of vaginal discharge and concomitant inflammatory changes involving the vervix and vagina. Although all of the affected mares recovered spontaneously, a high percentage continued to harbour the Gram-negative coccobacillus in their genital tracts for variable periods after challenge. Shedding of the organism was either relatively constant or intermittent and was not solely related to the oestrous period. Cytological examination of smears of cervical and urethral swabs was of diagnostic value only during the clinical phase of the infection. There was evidence that reinfection of mares could occur after an interval of 2 weeks.