Human red blood cells(erythrocyte)are suspended separately in several different kinds of ammonium solution whose osmotic pressure are equal to that of cells,and the speed at which ammonium molecules permeate the cell membrane are detected with 721A spectrometer.The results indicate that permeant speed is higher for weak acid ammonium than that of strong acid ammonium.The differences among various of ammonium which can enter into cells and the mechanism of their permeability are discussed from the view of electrolyte hydrolysis balance.