The quantitative estimation of the hepatic functional volume in rats was attempted using the serum dimethadione (DMO)/trimethadione (TMO) ratio in a single blood sampling after oral administration of TMO, which we call the TMO tolerance test, in order to develop a means of pre-operatively assessing hepatic resectability. Serum DMO/TMO ratios correlated well with the total amount of the hepatic microsomal TMO-N-demethylase activity (enzyme activity) and with the remnant liver weight after 37% and 68% partial hepatectomy. These ratios increased after partial hepatectomy in parallel with the changes in the enzyme activity and the remnant liver weight. The results suggest that the quantitative functional changes of the remnant liver after hepatectomy in both CCl4-treated and untreated rats can be estimated pre-operatively by the TMO tolerance test.