Paraquat dichloride at 250 ppm in the diet was fed continuously to rats. Though no apparent effect of paraquat was observed until 10 d, some rats then began to show several symptoms such as diarrhea, anorexia, epistaxis, and hypokinesia, and in some cases rats died after this period. The biochemical examination of plasma components revealed appreciable changes in the concentrations of an acute-phase reactant protein and some vitamins that act as antioxidants. alpha-Cysteine proteinase inhibitor increased by 5-fold, and vitamin C and its radical increased by 1.5- and 1.7-fold, respectively, whereas alpha 1 proteinase inhibitor decreased slightly. Paraquat enhanced the cysteine proteinase inhibitor levels in lung, liver, and kidney by 6.2-, 6.0-, and 4.5-fold of control, respectively. Among three components of alpha-cysteine proteinase inhibitor, the T kininogen level of treated rat plasma was about eight-fold higher than control, whereas the high-molecular-weight kininogen level was unchanged. The large increment of T kininogen was also seen in lungs of the treated rats.