Lesions of meningoencephalitis were found in 55 per cent of 372 rabbits comprising the laboratory stock regarded as healthy, others with snuffles or dying from different affections while being kept under observation, and still others which were employed for experimental purposes, such as tumor transplantation and Treponema pallidum inoculation. None was injected intracerebrally. The lesions consist in the main of infiltration with mononuclear cells occurring around the blood vessels, in the meninges, in the cortex, and under the ependyma of the lateral ventricles, together with particular focal necrotic areas in the cortex. The incidence of these histopathological changes varies in different series of animals; in those supposedly normal and in rabbits inoculated with a transplantable tumor or with Treponema pallidum material, the percentage of positives was from 40 to 60; in those suffering from miscellaneous diseases, such as pneumonia, septicemia, etc., the percentage was 70, and in rabbits ill with snuffles, as many as 76 per cent were affected. Marked lesions were observed in 47.5 per cent of the total.
The histopathological picture observed in these rabbits corresponds to those offered by a number of investigators as evidence of the transmission of certain nervous diseases of man to this animal. The accidental cerebral lesions in the rabbit, of a wide variety, and of frequent occurrence, are to be regarded as existing before any experimental procedure is begun. Their recognition is of the utmost importance in the interpretation of experimental results based on the presence of similar changes in this animal.