Aerobic chemostat cultures of Saccharomyces cerevisiae were performed at different dilution rates under energy (glucose) limitation or at various extents of energy excess imposed by a nitrogen limitation. It was found that energy excess induced metabolic uncoupling under steady-state conditions. The specific ethanol production rate was always higher the lower the feed medium nitrogen concentration throughout the whole range of dilution rates tested. In addition, the respiratory rate also increased under nitrogen limitation, for at least as long as the specific oxygen consumption rate was below the maximum capacity. These results imply that the ATP yield (Y-ATP in g biomass per mol ATP) and/or the amount of ATP produced, i.e. the P/O ratio, must be able to change. By assuming a constant Y-ATP of 16 g mol(-1), a decrease in the P/O ratio from 1 to 1.5 at low dilution rates and glucose limitation down to only 0.1-0.2 at high dilution rates and nitrogen limitation could be calculated. If instead a fixed P/O ratio of 1.0 was assumed, the Y-ATP decreased from about 20 down to below 10 g mol(-1) during these different conditions. Furthermore, the heat yield values, i.e. the amount of heat produced per amount of biomass formed, increased dramatically when the cells were subjected to a nitrogen limitation indicating less efficient growth in terms of conserving substrate energy as biomass under energy excess compared to energy limiting conditions.