The toxicity of emulsified methylparathion was examined in comparison with ethylparathion. i) The LD(50) of methylparathion given orally to mice was 19.4-30.5 mg/kg and its mean value was 25.4mg/kg. (LD(50) of ethylparathion was 11.5mg/kg) The approximate LD(50) of methylparathion (ethylparathion) for the rabbit was 40-50 (8-10) by subcutaneous injection, 50-60 (20) by oral route and 200-400 (50-100) by dermal application. Thus, methylparathion was about 1/2-1/3 as toxic as ethylparathion. ii) The manifestations produced by methylparathion administration for rabbits were similar to those by ethylparathion. In addition, it was noted that the rabbits Ìn moderately poisoned cases were led to death after a week from malnutrition accompanied with anorexia and diarrhea. iii) The inhibitory action of methylparathion on the rabbit blood cholinesterase in vitro, was about 1/2-1/3 as great as that of ethylparathion. iv) In the laboratory rabbit there occurced leucocytosis, mainly occupied by pseudoeosinophils, and the deflection of Arneth count to the left as well as relative lymphocytopenia was observed.