The changes in rat serum protein distribution after partial hepatectomy were examined using two-dimensional electrophoresis, utilizing isoelectric focusing in polyacrylamide gel in the first dimension and pore gradient polyacrylamide gel electrophoresis in the second dimension. Drastic decreases in amount of protein were observed at more than twenty spot positions, and drastic increases in amount or newly appeared proteins were observed at eight spot positions. The amounts of albumin, immunoglobulin M and alpha 2-macroglobulin did not change relatively after hepatectomy. The time course of the changes was examined using a densitometer, and it was observed that almost all the serum proteins changed drastically at 24 h after hepatectomy.