Trimethadione (TMO) is exclusively metabolized to dimethadione (DMO) by cytochrome P-450 dependent mono-oxygenase in liver microsomes. We have chosen TMO as an indicator compound for estimating the microsomal function of the liver. TMO tolerance test was performed in the CCl4 intoxicated injury or 68% partial-hepatectomized rats. Serum DMO/TMO ratios in 2 hours following oral administration of TMO in these rats in vivo were well correlated with changes in content of hepatic cytochrome P-450 and activity of TMO N-demethylase in vitro. TMO tolerance test could also determine quantitatively the changes of the functional capacity of the liver in the 68% partial hepatectomy. These results suggest that TMO tolerance test is a useful tool for the surgical indication in the hepatectomy.