Biochemical and gross pathological effects of diquat were studied with special attention to cysteine proteinase inhibitor level which was often increased in acute and chronic disorder. Diquat was fed continuously to rats at the dose of 1000 ppm in the diet. After 10 days, anorexia and severe diarrhea were observed but epistaxis and hypokinesia were not apparent. The rats were killed after feeding the diet for 13.5 days and plasma components such as acute phase reactant proteins and some vitamins which act as antioxidants were examined. The results showed that alpha-cysteine proteinase inhibitor (alpha-CPI) increased to 9-fold and vitamin C radical increased to 1.6-fold, whereas alpha 1 proteinase inhibitor (alpha 1-PI) decreased to 0.9-fold and vitamins C and E were the same as the control. Among three components of alpha-CPI, the T kininogen level in intoxicated rat plasma was about 20-fold, whereas the high molecular weight kininogen level was about 2-fold of the control. Diquat also enhanced the cysteine proteinase inhibitor (CPI) level to 20-fold in kidney and to 7- to 10-fold in the other organs. The large increment of T kininogen in these organs was also confirmed immunologically. The kidney showed a granular degeneration and its weight increased to 1.2-fold of control. The other organs showed neither gross pathological alteration nor weight change, compared with the control. The diquat distribution was highest in spleen and next highest in kidney among several organs. These results were compared with those caused by paraquat.