The metabolism of N-nitroso-N-methyl-N-(2-oxopropyl)amine was examined using freshly isolated hepatocytes from Fischer 344 rats. As determined by high performance liquid chromatography, it was found that the E isomer was preferentially metabolized when the parent mixture was used. When the two isomers were studied separately, the E isomer was efficiently metabolized in the hepatocytic system, whereas the Z isomer was not. The kinetics of disappearance of the Z isomer during metabolism was identical to that for the reequilibration of the Z isomer to the mixture of isomers in the absence of a metabolizing system.