The toxicity of Diazinon, Chlorthion and Sumicide are studied in comparison with parathion The approximate LD(50) for rabbit are (mg per kg of weight) By the application of these compound, the changes are as follows; 1) Blood cholinesterase of rabbit, both serum and blood corpuscle, are similarly inhibited as the case of Parathion. 2) Symptom of poisoning with rabbit is the same as Parathion. 3) By the rabbit, there occur leucocytosis, mainly due to pseudoeosinophils, and the Arneth count is deflected to the left. Lymphocyte shows relative decrease. 4) Frog heart is examined by Staub-Fuhner method. Heart block that can be made by over 10(-8) concentration of Parathion, also occurs by 10(-7) Diazinon and 10(-6) Chlorthion. 5) The blood vessels of rabbit ear are dilated by the application of 0.1mg Diazinon. 6) Diazinon and Chlorthion make the lowering of arterial blood pressure in the experiment with dog. 7) By the subcutaneous application of these compound the rabbit serum protein and albumin increase, and gamma-globulin decreases.