Naturally infected adult rats rarely have grossly observable changes. Experimentally infected 9–10 week old axenic rats develop gross lesions in the lung on postinoculation days 6 and 7, which consist of randomly dispersed red-brown to gray foci, less than 1 mm in diameter (Bhatt and Jacoby 1977). Although rat coronavirus may cause fatal pneumonia in a high percentage of newborn and day-old rats, gross pulmonary lesions have not been described (Parker et al. 1970).