1. 1. Glucose uptake was determined in soleus muscle after preincubation for 15 min in Krebs-Henseleit HCO3- medium containing 5 mM EDTA. Anoxia did not stimulate glucose uptake by these EDTA-treated muscles, unless insulin, together with Ca2+ and Mg2+, were added to the incubation medium. 2. 2. Whereas the permissive effect of insulin could be shown at concentrations below 10 μunits/ml, it was only above this concentration that insulin stimulated glucose uptake in normal muscle. 3. 3. When normal muscle was incubated aerobically in the absence of insulin, the initial rate of glucose uptake was maintained for only 1 h. Insulin at a concentration of 10 μunits/ml, which did not affect this rate, prolonged the linearity for the entire 2-h incubation period. 4. 4. From this it is proposed that insulin at low concentrations exerts a "priming" effect on the cell, which would correspond to the stabilizing action often attributed to the hormone. Stimulation of sugar transport by higher concentrations of insulin thus represents a secondary action of the hormone. © 1970.