The authors made a report on one case which had carebral hemorrhage and residual motoric aphasia after using an insecticide, tetrachloro-ethylene, and on two cases with mental weakness mainly consisting of persistent systemic emaciation, hypersensitivity, and deficint thinking and memory. In the observations of the brain and liver of mature Rattus injected with tetrachloro-ethylene (5cc/kg), the congestion of blood was observed in the blood capillaries of the brane as a whole, especially around the ventricles the edematous part and many hemorrhagic foci were recognized. In the liver there was an atrophic tendency of parencymal cells, and hemorrhagic foci occasionally in the Disse cavity and Glisson's sheath, all these being only very slightly. In the in vitro experiment to see the effects of addition of vitamins B(1), B(2), panthotenic acid, co-carboxylase, glucuronic acid, d. l-methionine, and cystein on the tissue respiration of the liver and brain of the rats intoxicated by tetrachloro-ethylene, cystein brought a marked rise of XO(2) in the liver and d. 1-methionin slightly. None of these, however, seemed to improve the fall of XO(2) in the brain. Clinically the use of tetrachloro-ethylene to the person of advanced age or to those who have disturbances in the liver and blood vessels are dangerous, especially its use is prohibitive to those with a distinct arterial sclerosis.