The use of rabbits for the experimental study of the virus of epidemic (lethargic) encephalitis and allied conditions has been instrumental in directing attention to peculiar brain lesions, which were first noted in these animals by Bull1in 1917 and by Oliver2in 1922. These lesions are disturbing factors for the reason that, although no signs of disease can be detected clinically, they may be found in about 60 per cent, of ordinary stock rabbits.3The observation that we wish briefly to report is that the lesions in question are not confined to rabbits, but occur also in mice which likewise show every appearance of being healthy. Infiltrative lesions precisely like those described by Bull and Oliver have been found by us in the brains in twenty-five out of 141 mice examined histologically. They were of the characteristic meningeal, perivascular, focal and subependymal types. As in the