Theoretical calculations of the NADPH requirement for biomass formation indicate that in yeast this parameter is strongly dependent on the carbon and nitrogen sources used for growth. Enzyme surveys of NADPH-generating metabolic pathways and radiorespirometric studies demonstrate that in yeast the HMP pathway is the major source of NADPH. Furthermore, radiorespirometric data suggest that in yeasts the HMP pathway activities are close to the theoretical minimum. It may be concluded that the mitochondrial NADPH oxidation, which in yeasts may yield ATP, is quantitatively not an important process. The inability of C. utilis to utilize the NADH produced in formate oxidation as an extra source of NADPH strongly suggests that transhydrogenase activity is absent. Furthermore, the absence of xylose utilization under anaerobic conditions in most facultatively fermentative yeasts indicates that also in these organisms transhydrogenase activity is absent. This conclusion is supported by the observation that anaerobic xylose utilization is observed only in those yeasts which possess a high activity of an NADH-linked xylose reductase. Hence in these organisms the redox-neutral conversion of xylose to ethanol is possible, since the second step in xylose metabolism is mediated by an NAD+-linked xylitol dehydrogenase.