The action of puromycin and cycloheximide on the uptake of D-xylose in isolated frog sartorius muscles has been studied. Cycloheximide and puromycin were found to stimulate the basal transport of xylose and to inhibit the insulin-stimulated sugar uptake. On the other hand, both these antibiotics did not affect the sugar transport stimulated by 2,4-dinitrophenol. Both of them, even at a high concentration (up to 0.1 mg/ml), inhibited the incorporation of leucine or valine into muscle proteins by 50% only, whereas they did inhibit the sugar transport at a concentration much lesser by several orders. The inhibitors of protein synthesis are supposed to affect directly the signal transmission from insulin receptors to the sugar transport system.