The glycogen synthase system in skeletal muscle obtained by needle biopsy was studied in normal individuals and in patients with myotonic dystrophy both before and after either oral glucose or intravenous insulin administration. Glycogen phosphorylase activity was also determined. Total synthase activity in the myotonic dystrophy patients was similar to the controls when expressed per milligram soluble protein. The initial per cent of total synthase in the I form was also similar. Following glucose or insulin administration, there was a significant increase in the per cent synthase I in the normal females, but not in the normal males. In patients with myotonic dystrophy, the response was reduced and more variable, and a sex difference was not detectable. Why insulin stimulates a greater conversion of synthase D to synthase I in normal females than in normal males is not understood at present. In patients with myotonic dystrophy, total phosphorylase and the per cent of phosphorylase in the a form were similar to those in normal individuals. There was a modest decrease in per cent of phosphorylase in the a form after glucose or insulin administration in the myotonic dystrophy patients, but not in the normals. This is of doubtful physiologic significance.