The Lactic Dehydrogenase(LDH) and its isoenzyme activities in Cerebrospinal Fluid(CSF) of 50 patients with Cental Nervous System (CNS) disease were studied to explore the possibility of difference among the various meningitis and their relationshiop to its prognosis, and to compare with 12 patients without CNS infection as control group The results obtained were as follows : 1) The levels of total LDH, LDH 1 and LDH 5 in control group were within normal limit of 18.3 units, 51.8% and 0% respectively. 2) The LDH activities were highest at purulent meningitis group, mederate at T.B. meningitis group and lowest at aseptic meningitis group. 3) Prognosis was poor in the group with highly elevated LDH activities and LDH5 values, which included purulent meningitis and T.B. meningitis accompanied by unconsciousness, and good prognosis was noted with normal LDH activities in aseptic meningitis. 4) The LDH activities and LDH5 values of the partially treated purulent meningitis group were significantly elevated compared to those of T.B. and aseptic meningitis groups 5) The LDH activities became normalized in a week of appropriate treatment for purulent meningitis and in 7 weeks treatement for T.B. meningitis.