The Na/K ratio is well known to generate and control neural signals. We present here results on studies of the effect of Na/K ratio on rigidity or compressibility (κ) of Langmuir monolayers of Dipalmitoyl Phosphatidylcholine (DPPC), a major constituent of cell membranes. A considerable increase in κ (κNa) is seen with 10-2N Na+ ions in the aqueous subphase, while the increase with 10-2N K+ ions (κK) is marginal. For a Na/K = 1 mixture of the same overall ion concentration,κK>κ>κNa but it lies closer to κNa. For Na/K = 2/3 the increase in κ over the equimolar value is very small whereas for Na/K = = the κ value is very close to κK. These results indicate a new role of Na/K ratio, that of controlling the fluidity of the cell membrane. © 2020 American Institute of Physics Inc.. All rights reserved.