Parathion, though mainly having a function as nervous poison, has been investigated for its effect on the liver. By having taken resort to reactions such as follows as to both experimental animals as well as patients so many as 43, including 20 mild cases, 11 moderate and 12 serious, i.e., Takata's reaction, cobalt reaction, Gros' test, Sawada's test, Thymol turvidity test, Cephalin-cholesteral flocculation test, Weltmann's reaction, Meulengradt's test, H.v.d. Bergh's test on serum, bilirubin, urobilin and urobilinogen test on urin, bromsulphalein test, santonin test, galactose test, and pathological anatomical examination, was proved the following facts; those results indicate no other disturbances except positive urobilinogen reaction. This even may probably be supposed to have turned positive due to irritating of parasympathicus. There was no room to consider mishaps such as liver swelling or jaundice. From clinical aspect, therefore, there seems to be no need for any consideration about liver disturbance.