The importance of the physiological state for the catabolic capacity during carbon- and energy-starving conditions was studied. Endogenous metabolism was low in cells depleted of carbon and energy source. This does not necessarily mean that these cells do not have the capacity for a higher catabolic activity. To measure the catabolic capacity of starved cells, carbon- and energy-depleted cells were suspended in synthetic fresh water and the respiratory and fermentative rates were examined after addition of glucose. The catabolic capacity was studied in cells of different physiological states. Stationary phase cells, which were depleted of their carbon and energy source for 6 h, showed a lower respiratory capacity, but almost as high a fermentative capacity as cells originating from the logarithmic phase of growth on glucose. With extended starvation time of stationary phase cells, the fermentative capacity decreased, while the respiratory capacity increased. Transition phase cells, characterised by a metabolic shift from a mixed respiratory-fermentative catabolism to a purely respiratory catabolism, showed, when newly harvested, a lower fermentative capacity than log phase cells with a mixed respiratory-fermentative catabolism. However, the fermentative capacity decreased during starvation of carbon and energy source for log phase cells, whereas transition phase cells increased the fermentative capacity to the same level as that of newly harvested log phase cells after 4 days without carbon or energy source. The addition of glucose to stationary phase cells under non-growth conditions (no nitrogen source) resulted in ATP production rates of between 50% and almost 100% of the ATP produced by newly harvested log phase cells under the same conditions. Much higher rates of ATP production were attained for these different types of physiological states than can be explained by maintenance energy requirements. The suggested explanation for this over-consumption of energy during non-growth conditions in response to energy excess is metabolic uncoupling. This behaviour is discussed in relation to maintenance energy requirements during different environmental conditions.