Changes in transmembrane potential and ionic contents of rat alveolar macrophages were studied. Alveolar macrophages were harvested from male Long-Evans-rats by tracheal lavage and cell pellets were obtained by centrifugation through dibutyl-phthalate doped with mineral oil. The cell volume, cell water content, transmembrane potential, and the intracellular concentrations of sodium, potassium, and chloride ions were measured. The cell water content was approximately 72 percent and the mean cell volume of the alveolar macrophages was approximately 1,525 cubic microns. The transmembrane potential was calculated to be approximately minus 37 millivolts with the cytoplasm being negative with respect to the extracellular fluid. Intracellular fluid concentrations were approximately 96 millimoles of sodium, 50 millimoles of potassium, and 64 millimoles of chloride. Neither sodium, potassium, or chloride was distributed at equilibrium. The authors conclude that rat alveolar macrophages contain more sodium than potassium unlike other rat cells. They suggest that transmembrane potential plays an important role in the function of phagocytic cells.