As far as the pathogenesis of poisonings with organophosphorus pesticides is concerned, in addition to irreversible inhibition of acetylcholinesterase (AGE) in tissues, of importance are changes in the other systems which essentially determine the outcome of intoxication. The purpose of the present study was to examine the nature of changes occurring in total protein and protein fractions, free amino acids (aspartic and glutamic acids, glycine, isoleucine, leucine) and in certain enzymes (AST, ALT, CP, GGTP, GDH) in the cerebrospinal fluid (CSF) of patients with acute Malathion insecticide poisoning. 137 patients aged 20 to 50 years were placed under observation. There were 77 men and 60 women. 40 persons had poisoning of medium gravity and 97 were severely poisoned. The intake of the CSF was performed on days 1, 3, 10, 14 and 21 since the disease onset. It has been established that in acute Malathion insecticide poisoning, the CSF content of the stimulating mediator amino acids, aspartic and glutamic, rises within the early periods, whereas the concentration of the inhibitory mediator glycine decreases. The changes in protein fractions of the CSF are characterized by a fall of the content of globulins and a rise of albumins, thus attesting to the predominance of pathological processes in the brain, especially in the initial period of intoxication, and to the impairment of the blood-brain barrier. The development of intoxication is associated with activation in the CSF of LDN, CP, GGTP and GDH as well as by activation of LDH isozymes which is viewed as the result of the membranotoxic effect of a Malathion insecticide.