The cellular energy state as well as ureogenesis and gluconeogenesis as ATP-consuming processes were studied as a function of hydrogen supply of mitochondria in isolated hepatocytes. Even though the ATP turnover was quite different during ureogenesis and gluconeogenesis and in the absence of these syntheses both the safranine signals (as a probe for the mitochondrial membrane potential) and the total ATP contents of the cells did not vary to a great extent. Partial uncoupling by FCCP destroyed the mitochondrial membrane potential and decreased the cellular ATP totally. Under these conditions the presence of ornithine plus lactate maintained a measurable membrane potential and ATP level. In order to demonstrate the relevance of hydrogen supply of mitochondria to the syntheses of glucose and urea, the load on energy metabolism was additionally increased by partial uncoupling, and the cells were incubated in a parallel mode in Krebs-Henseleit buffer and an 80 mM succinate containing medium, the latter forcing succinate into the cells. After partial uncoupling, ureogenesis and gluconeogenesis as well as the ATP level were seen to decrease markedly in Krebs-Henseleit buffer, whereas in the succinate medium the improved hydrogen supply of mitochondria counteracted the drain by FCCP, and substantially higher rates of the syntheses and elevated ATP levels were maintained.