1. The action of insulin on the transport and the distribution of Na and K has been studied in rat soleus muscles incubated at 30 degrees C in glucose-free Krebs-Ringer bicarbonate buffer. 2. Measurements of the uptake and the wash-out of 22Na indicate that the muscles contain an intracellular pool of Na available for transport which is confined to the water space not available to sucrose. Ouabain (10(-4)-10(-3)M) inhibited 22Na efflux by 69% (0-287 micronmole/g tissue wet weight per minute) and 42K-influx by 40% (0-196 micronmole/g tissue wet weight per minute). When all extracellular Na was replaced by Li, both 22Na-efflux adn 42K-influx were inhibited to about the same extent and ouabain produced very little further inhibition. 2,4-dinitrophenol decreased the ouabain-resistant component of 22Na-efflux by 39%. 3. Insulin (from 0-1 to 100 mu./ml.) increased the rate coefficient of 22Na-efflux by from 11 to 46% within 15 min. In the presence of ouabain (10(-3)M), the same relative increase was obtained, indicating that the hormone stimulates the glycoside-sensitive and the glycoside-insensitive Na transport to a similar extent. The effect of insulin on 22Na-efflux was not abolished by tetracaine (0-5 X 10(-3)M), phlorizin (0-5 X 10(-2)M) or by the substitution of Na, K, Mg or Ca. In the presence of 2,4-dinitrophenol (0-5 X 10(-4)M) or at temperatures below 15 degrees C, the hormone produced no detectable change in 22Na-efflux. 4. Insulin increased 42K-influx from 0-525 to 0-664 mumole/g tissue wet weight per minute. This effect was entirely blocked by ouabain but not by tetracaine. Insulin produced a 14% transient decrease in 42K-efflux. 5. The continued exposure to insulin led to a new steady state, in which the intracellular Na pool was decreased from around 10 to around 5 mumole/g tissue wet weight and the K content increased by an equivalent amount. In the presence of ouabain or at low extracellular concentrations of K, insulin increased the rate of 22Na-influx by around 35%. This effect was blocked by 2,4-dinitrophenol but not be tetracaine. 6. It is concluded that insulin stimulates the active coupled transport of Na and K, possibly by increasing the relative Na-affinity of the system mediating this process.