Introduction Concussion is a transient reversible alteration of the central nervous system of immediate onset resulting from an impact to the head. It is defined physiologically by a temporary abrogation of certain reflex functions, and pathologically by the absence of gross destruction of brain substance, but presence of finer cellular changes; both psysiological and pathological evidences indicate a particular affection of the brain stem. Denny-Brown and Russell5distinguished an acceleration concussion of the freely movable head from a compression concussion of the rigidly fixed head. The present investigation is limited to the neuropathology and mechanics of experimental acceleration concussion in cats; the relationship of acceleration concussion to compression concussion will be discussed on the basis of these findings. Two experimental series are reported in this article: the first part describes the histopathology of cats with concussions compared to the symptoms and the mechanical forces involved. The second part describes