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1. Urea and water permeabilities of chicken erythrocytes are considerably lower than those of mammalian red cells. 2. The permeabilities to urea, thiourea and to N-methylurea (about 10(-6) cm/sec at 25 degrees C) were independent of concentration within a very broad range, and we found no evidence of interaction between transport of analogue molecules. The activation energies were between 17 and 19 kcal/mole, and urea transport was not inhibited by phloretin, which inhibits urea transport in mammalian red cells. 3. The water permeability of chicken red cells (as measured by the diffusion of tritiated water) was 1-35 X 10(-3) cm/sec at 25 degrees C. The activation energy was 10 kcal/mole, and the water permeability was not affected by phloretin or parachloromercuribenzoate. 4. It is concluded that the urea and water permeabilities of the chicken erythrocyte membrane are similar to those of a non-porous bimolecular phospholipid membrane. 5. Like the red cells of other animal species the chicken red cell membrane contains an anion transport system, mediating a rapid exchange of chloride across the cell membranes. The pH dependence, temperature dependence, and sensitivity to inhibitors were similar to the properties of the anion transport system found in mammalian red cells. Our study shows, therefore, that the transport system offers a highly specific pathway to the exchange of anions, without presenting an inspecific leak to the permeation of water and urea.

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