The osmotic permeability coefficient for water has been measured for the Ehrlich mouse ascites tumor cell. Measurements were made of the rate of cell shrinkage in hyperosmotic solutions of NaCI, a functionally impermeable solute. During the first 9 months of weekly serial transplantation the mean was 6.4 µ(3)/µ(3)/atm. ± 0.8 (S.E.). By the end of the 2nd year the permeability coefficient was much lower and averaged 1.6 ± 0.09. There were no significant differences in the volume of the tumor cells which could explain the discrepancy on the basis of a change in the volume to surface area ratio. Studies of the effect of temperature were done and Eyring's theory of absolute reaction rates was applied to the data. The apparent energy of activation was 9.6 kcal./mol and ΔS‡ was 39.1 entropy units. The thermodynamic data are twice as high as data reported by Wang for self-diffusion and viscous properties of water. Two alternate explanations have been advanced based on the pore hypothesis of membrane permeability. One explains the thermodynamic data from a change in the A'/Δx available for water movement; the other assumes A'/Δx constant and bases the results on the interaction of water dipoles with each other and the membrane
