The change of catechol-O-methyltransferase (COMT) activity during regeneration of rat liver was studied. Cytosolic, mitochondrial and microsomal COMTs activities were estimated in regenerating rat livers over a period of ten days after $70\%$ (median and left lateral lobes) partial hepatectomy. The values of Km and Vmax in the hepatic enzymes were also measured. The activities of cytosolic and microsomal COMTs in regenerating rat liver after partial hepatectomy were found to be significantly increased between the second and the third day. Whereas the mitochondrial COMT activity did not change. The Vmax values of the cytosolic and microsomal COMTs in the regenerating rat liver were significantly increased at the second day after partial hepatectomy, however, the Km values of the above hepatic enzymes did not vary in all the experimental groups. Therefore, the results suggest that the biosynthesis of COMT was increased during the regeneration of rat liver.