1. The effect of a redox cycler and arylator (menadione) and a pure arylator quinone (benzoquinone) was studied on different NADPH generating and consuming processes in isolated mouse hepatocytes. 2. Menadione inhibited gluconeogenesis from alanine but not from fructose or glycerol. 3. Drug oxidation measured as aniline hydroxylation and aminopyrine N-demethylation could be inhibited by menadione in microsomal membrane and in isolated hepatocytes both from fed or fasted animals. 4. Ureogenesis in isolated hepatocytes from fed mice could not be inhibited even by high concentration of menadione, while in cells from fasted animals menadione was inhibitory at high concentration in the presence of gluconeogenic precursor and at lower concentration in the absence of it. 5. Benzoquinone did not inhibit the above mentioned processes.