Earlier studies on inflammation in the CNS have largely focused on conditions with an immune component. Recent evidence has emerged, however, that the innate, acute inflammatory response in the CNS parenchyma is quite unlike that in other tissues. The meninges and ventricular compartments show more typical responses, as does the parenchyma of the brain in immature animals. It is becoming apparent that the cells of the mononuclear phagocyte lineage dominate inflammatory responses in the CNS parenchyma.