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The temperature dependence of permeation across human red cell membranes has been determined for a series of hydrophilic and lipophilic solutes, including urea and two methyl substituted derivatives, all the straightchain amides from formamide through valeramide and the two isomers, isobutyramide and isovaleramide. The temperature coefficient for permeation by all the hydrophilic solutes is 12 kcal mol -' or less, whereas that for all the lipophilic solutes is 19 kcal mol -' or greater. This difference is consonant with the view that hydrophilic molecules cross the membrane by a path different from that taken by the lipophilic ones. The thermodynamic parameters associated with lipophile permeation have been studied in detail. AG is negative for adsorption of lipophilic amides onto an oil-water interface, whereas it is positive for transfer of the polar head from the aqueous medium to bulk lipid solvent. Application of absolute reaction rate theory makes it possible to make a clear distinction between diffusion across the water-red cell membrane interface and diffusion within the membrane. Diffusion coefficients and apparent activation enthalpies and entropies have been computed for each process. Transfer of the polar head from the solvent into the interface is characterized by AGI = 0 kcal mol-' and AS S negative, whereas both of these parameters have large positive values for diffusion within the membrane. Diffusion within the membrane is similar to what is expected for diffusion through a highly associated viscous fluid.Permeability coefficients for the passage of small hydrophilic and lipophilic solutes across red cell membranes have been determined by Sha'afi et al. (1) who concluded that physical chemical interactions between solute and membrane were the basic determinants of red cell permeability for both classes of solutes. We have measured the temperature dependence of permeation for typical members of these hydrophilic and lipophilic classes of solutes in order to gain further insight into the nature of the interactions with the red cell membrane structure.The temperature coefficients for permeation by hydrophilic solutes are significantly lower than those for lipophilic ones, thus providing an additional

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