A method is described for the perfusion of the rat kidney with a medium resembling rat plasma in its ionic constituents but with 3% polyvinylpyrrolidone (mol.wt. 44,000) replacing the plasma proteins. 2. In the early stages of perfusion, there was rapid swelling of the tubular cells in the outer medulla, a low glomerular filtration rate and a low filtration fraction. 3. After 30 min perfusion, the tubular cell volume in the outer medulla was normal and filtration had increased and stabilised. 4. Cell swelling and the low initial filtration fraction were prevented by (a) adding 50 mM-mannitol to the basic medium, (b) replacing 50 mM-NaCl with 50 mM-choline chloride, or (c) adding 5% human serum to the basic medium. 5. The addition of human serum to the perfusion medium depressed the fractional reabsorption of sodium and potassium; sodium from 0.94 +/- 0.01 (S.E. of mean) with the basic medium to 0.79 +/- 0.02 with basic medium +5% serum after 35 min perfusion; and potassium from 0.73 +/- 0.04 with the basic medium to 0.37 +/- 0.05 with the basic medium +5% serum after 35 min perfusion. 6. Two conclusions are drawn from the observations: first, that at the start of perfusion the effective internal osmolality of tubular cells of the outer medulla is greater than that of cortical cells and of the basic medium; and second, that the presence of small amounts of plasma proteins in the perfusion medium has a marked effect on the properties of plasma cell membranes.
