The apparent activation energy for the water diffusion permeability coefficient, Pd, across the red cell membrane has been found to be 4.9 -0.3 kcal/mole in the dog and 6.0 4-0.2 kcal/mole in the human being over the temperature range, 7 to 37°C. The apparent activation energy for the hydraulic conductivity, L,, in dog red cells has been found to be 3.7 0.4 kcal/mole and in human red cells, 3.3 4 0.4 kcal/mole over the same temperature range. The product of Lp and the bulk viscosity of water, w, was independent of temperature for both dog and man which indicates that the geometry of the red cell membrane is not temperature-sensitive over our experimental temperature range in either species. In the case of the dog, the apparent activation energy for diffusion is the same as that for self-diffusion of water, 4.64.8 kcal/mole, which indicates that the process of water diffusion across the dog red cell membrane is the same as that in free solution. The slightly, but significantly, higher activation energy for water diffusion in human red cells is consonant with water-membrane interaction in the narrower equivalent pores characteristic of these cells. The observation that the apparent activation energy for hydraulic conductivity is less than that for water diffusion across the red cell membrane is characteristic of viscous flow and suggests that the flow of water across the membranes of these red cells under an osmotic pressure gradient is a viscous process.The physical state of the water in biological membranes is a matter of fundamental significance which may be studied by measurement of the temperature dependence of water fluxes across cellular membranes. Hays and Leaf (1) found the apparent activation energy for the diffusion of tritiated water (THO) across the unmodified membrane of the isolated toad bladder to be 9.8 kcal/mole. This figure is much larger than the value of 4.6 kcal/mole obtained by Wang et al. (2) for the diffusion of THO in bulk water. In studies of THO diffusion in peripheral nerve fibers, Nevis (3) computed apparent 45'

Sodium transport and oxygen consumption were studied simultaneously in the short-circuited frog skin. Sodium transport was evaluated from Io/F, where I is the short-circuit current measured with standard Ringer's solution bathing each surface and F is the Faraday constant. Oxygen tension was measured polarographically. Under a variety of circumstances the rate of oxygen consumption from the outer solution exceeded that from the inner solution, the ratio being constant (0.57 4 0.09 SD). Both 1, and the associated rate of oxygen consumption J,, declined nonlinearly with time, but the relationship between them was linear, suggesting that the basal oxygen consumption was constant. For each skin numerous experimental points were fitted by the best straight line. The intercept (Jr o )l,.o then gave the basal oxygen consumption, and the slope dNa/dO 2 gave an apparent stoichiometric ratio for a given skin. The basal oxygen consumption was about one-half the total oxygen consumption in a representative untreated short-circuited skin. Values of dNa/dO2 in 10 skins varied significantly, ranging from 7.1 to 30.9 (as compared with Zerahn's and Leaf and Renshaw's values of about 18). KCN abolished both I and J,. 2,4-dinitrophenol (DNP) depressed I1 while increasing J, four-to fivefold. Antidiuretic hormone stimulated and ouabain depressed both I, and J,o; in both cases apparent stoichiometric ratios were preserved.

Studies were made of the dependence of the rate of oxygen consumption, J , on the electrical potential difference, A,, across the frog skin. After the abolition of sodium transport by ouabain the basal oxygen consumption was independent of /x. In fresh skins J, was a linear function of Alk over a range of at least --70 my. Treatment with aldosterone stimulated the shortcircuit current, Io, and the associated rate of oxygen consumption, Jro, and increased their stability; linearity was then demonstrable over a range of 4-160 mv. Brief perturbations of A4 (30-200 my) did not alter subsequent values of 10. Perturbations for 10 min or more produced a "memory" effect both with and without aldosterone: accelerating sodium transport by negative clamping lowered the subsequent value of I0; positive clamping induced the opposite effect. Changes in Jo were more readily detectable in the presence of aldosterone; these were in the same direction as the changes in I0. The linearity of J. in A& indicates the validity of analysis in terms of linear nonequilibrium thermodynamics-brief perturbations of A#p appear to produce no significant effect on either the phenomenological coefficients or the free energy of the metabolic driving reaction. Hence it is possible to evaluate this free energy.