1. The effect of extracellular potassium on the transport of sodium and potassium in rat thymocytes has been studied in vitro.

2. A significant increase in the rate constant for total and ouabain-sensitive sodium efflux was demonstrated at an extracellular potassium concentration of 1 mmol/l as compared with that at either 0 or 2 mmol/l.

3. At potassium concentrations below 3 mmol/l ouabain-sensitive sodium influx was observed suggesting sodium-sodium exchange catalysed by the sodium pump.

4. Both total and ouabain-insensitive potassium efflux rose with external potassium. A small ouabain-sensitive potassium efflux was observed at all levels of external potassium studied.

5. Total and ouabain-insensitive potassium influx increased with external potassium, but did not appear to saturate. Ouabain-sensitive potassium influx reached a maximum at an external potassium concentration of 2 mmol/l then decreased with increasing external potassium.

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