Changes in rat plasma proteins after administration of paraquat (PQ) were observed by micro two-dimensional polyacrylamide gel electrophoresis (Type II), under denaturing conditions. In the area of molecular weight lower than that of albumin, there were increases in four spots of plasma proteins, i.e., those at isoelectric points (pI) 5.0 and 5.9 with a molecular weight (MW) of ca. 54, 000, and 5.7 and 7.5 with a MW of ca. 15, 000, in the rats administered PQ. On the other hand, there were decreases in two spots of plasma proteins, i.e., those at pI 5.0-6.0 with a MW of ca. 54, 000 and 5.0 with a MW of ca. 18, 000. The protein spot at the pI 5.0 with a MW of ca. 18, 000 was identified as α2u-globulin (AUG) by MALDI-TOF/MS/MS.Some rats administered PQ showed the marked decrease in AUG, and pathological findings of pulmonary fibrosis were also observed. These results suggest the involvement of AUG with PQ-induced pulmonary fibrosis.