NADPH and NADH mediate reductant flow between cellular processes, linking central carbon and energy metabolism with intermediary metabolism, stress defence and development. Recent investigations have revealed paths of functional interactions, and have suggested that mitochondrial NADPH oxidation, especially together with the oxidative pentose phosphate pathway, is an important regulator of the cytosolic NADPH reduction level. Furthermore, stress-dependent metabolic pathways substantially affect the NADPH reduction level in particular physiological situations. The mitochondrial impact on the NADPH reduction level provides a model example of the physiological significance of the mitochondrial NAD(P)H dehydrogenase set-up, which is more complex in plants than in other organisms.