The histopathological response of male ICR mice to yessotoxin, isolated from the digestive organ of scallops Patinopecten yessoensis, was compared with that of desulfated yessotoxin. The target organ of the former was the heart and those of the latter were the liver and pancreas. Electron microscopically, marked intracytoplasmic edema in cardiac muscle cells was seen within 3 hr after the i.p. injection of over 300 micrograms/kg of yessotoxin. In contrast, desulfated yessotoxin at the same dose caused within 24 hr of i.p. injection severe fatty degeneration and intracellular necrosis in the liver and pancreas but not in the heart. Biochemically, the content of triglycerides in the liver of mice treated with desulfated yessotoxin increased about 60 times, and phospholipids two-fold more than the control levels of those of mice treated with yessotoxin.