For the systematic study of the role of inflammation in the morbidity and mortality associated with bacterial meningitis, techniques for quantitation of the inflammatory reaction in the meninges of rabbits with experimental pneumococcal infection were developed. The brains of 19 infected animals were removed intact, and the area of inflammation in microscopic sections was quantitated by an electronic X-Y plotter connected to a computer. Exudate was maximal along the ventral surface of the brain at the level of the cerebellum. Inflammation increased progressively with time and peaked at 72 hr. In a separate group of 29 animals, lactic acid dehydrogenase concentrations in cerebrospinal fluid increased significantly during infection, and the rate of increase wirh time coincided with the increase in inflammation documented histologically. The described method of quantitating inflammation in the meninges during experimental meningitis makes it possible to study the increase in granulocyte involvement with time. The establishment of a direct relation between the concentration of lactic acid dehydrogenase in the cerebrospinal fluid and the inflammatory mass validates the use of lactic acid dehydrogenase as an indicator of inflammation.