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Department of Physiology, University of Otago Medical School, Dunedin, New Zealand

1. Cortical slices from kidneys of adult rats were incubated for 1 and 2 hr at 25 °C in oxygenated solutions containing 5 mM-K, 2·5 mM-Ca and Mg, 0·007 M-phosphate buffer (pH 7·4) and 5 mM-glucose, together with 1, 2, 4 and 6% polyethylene glycol (PEG 6000) and 46, 92, 123, 154 and 231 mM-NaCl.

2. There were no significant differences in water content between slices incubated for 1 and for 2 hr.

3. The slices contained more water when the concentration either of NaCl or of PEG 6000 was lower.

4. Larger increases in the contribution of NaCl to total osmolarity were required to prevent increases in water content accompanying relatively small reductions in the contribution of PEG 6000. At a physiological salt concentration around 300 m-osmole/l., it took 10 m-osmole/l. of additional NaCl to compensate for the loss of 1 m-osmole/l. PEG 6000 and keep the volume of cell water normal.

5. The much greater contribution, osmole for osmole, which PEG 6000 made to tonicity suggests that metabolic activity failed to make the cell membranes effectively semipermeable; they were still substantially permeable to NaCl.

6. Comparison with earlier results suggested that ionic strength affected colloid osmolarity similarly in respiring and in non-metabolizing cells; and that respiration at 25 °C somehow kept the concentration of diffusible solutes in the cells sufficiently lower than outside to compensate for a swelling effect of intracellular colloid osmolarity which, in the absence of metabolism, could be balanced by 20 m-osmole/l. PEG 6000 added to the medium.