The general goal of this project is to demonstrate that fluorometric methods may be used to monitor membrane potentials in a eucaryote cell, the Ehrlich ascites tumor cell. We are studying the membrane potential in conjunction with measurements of passive ion fluxes, co-transport and active ion transport, in order to develop a quantitative expression for the potential difference across the membrane of this cell. These measurements, and similar studies with membrane bound cytoplasts of the cells, are being used to test the hypothesis that the energy for Na-dependent amino acid transport is derived solely from the electrochemical gradient of Na ion (the Na ion gradient hypothesis). We are developing the cytoplast preparation to avoid the complications of assessing the Na ion gradient if the nucleus sequesters Na ion. The influence of the membrane potential is also being considered in other aspects of Na ion-dependent amino acid transport, including stoichiometry and the possible involvement of K ion.