Two strains of CAR bacillus from a mouse (CB-M) and a rat (CB-R) which were passaged 11th in embryonated chicken eggs via the allantoic route were inoculated intranasally in ICR mice and Wistar rats. The histopathological changes and the localization of the CAR bacillus in the tracheas and lungs of these animals were investigated microscopically 2, 4 and 8 weeks postinoculation (PI). Histopathological changes similar to those in natural cases of CAR bacillus infection, showing severe peribronchial lymphoid cuffing, were first recognized 4 weeks PI. CAR bacillus was also found on the cilia of the respiratory epithelium. These histopathological changes were more remarkable in mice inoculated with CB-M and in rats inoculated with CB-R.