Glycogen synthesis was studied in rat tongue striated muscle after administration of 3H-glucose using light and electron microscope radioautography. Neither fasting nor addition of unlabeled glucose were needed to produce incorporation of 3H-glucose into glycogen, the only radioactive substance found in the tissue. Intense radioautographic reactions indicating glycogen synthesis were already seen 20 minutes after injection and were preferentially located over glycogen accumulations in the intermyofibrillar spaces. A striking variation of silver grain density from fiber to fiber, was a somewhat unexpected finding in a muscle consisting of fibers of uniform metabolic type. Decrease in grain counts at the 90-minute interval suggested a rapid turnover of muscle glycogen. In agreement with this, there were no morphological signs of glycogen accumulation on glucose administration. PA-Schiff staining, as well as the number of glycogen granules in electron micrographs, did not increase, nor did granule diameters change significantly during the experiment. Electron microscope radioautographs showed all silver grains to be located over or very near glycogen granules and sarcoplasmic spaces. While all newly-formed glycogen seemed associated with preexisting granules, there was no evidence for the participation of the sarcoplasmic reticulum in glycogen synthesis.