The transmembrane chemical gradient for Na+ that most living cells maintain represents an energy source which is not sufficient to account for observed cellular sugar or amino acid gradients. This short review summarizes the literature pointing to the above conclusion, and described experimental evidence from a variety of model systems which indicates that the membrane potential can provide an additional driving force. Ordinarily, solute leak pathways compromise the full gradient forming capability of the Na+-dependent concentration systems and obscure the extent to which membrane potentials are important. When these "leaks" are experimentally controlled, solute gradients are established that identify the membrane potential as a quantitatively very important energy input. Furthermore, if the electrochemical gradient for Na+ is the sole source of energy, an extremely high efficiency of energy transduction must occur during gradient-coupled transport. An experimental approach is described here which can provide clues to the mechanistic role for membrane potentials in intestinal Na+-dependent sugar transport.