A new method is described to distinguish between life and death by means of permeability to Na24. By exposing excised live ovarian eggs of Rana pipiens to radioactive Ringer soln., it was found that an avg. egg would exchange 2.6 x 10-7 g. of Na after one hour. However dead cells exchanged up to 7 times this amt. in only 10 min. A new method of making radioautographs of single cells frozen in liquid air showed that, after 30 min., the Na was evenly distributed throughout the cytoplasm, but at 2, 4, 6 and 8 min. of exposure "ring" pictures of increasing depth were found. Calculations based on Geiger counter measurements as well as on the radioautographs gave a value of 2.6 x 10-7 sq. cm./sec. for the diffusion constant of Na in cytoplasm. Since dead eggs concentrated Na24 up to 40 x 10-7 g. Na per cell (above the level of 25.8 x 10-7 g Na per egg as determined chemically) it was concluded that 2 different types of binding limit the diffusability of sodium within the cell. Only 12% of the normal Na was readily exchangeable. The remainder exchanged very slowly.