Quantitative biochemical analysis of glycogen was performed with and without perchloric acid pretreatment on biopsy specimens of varying glycogen content, obtained at rest and immediately after exercise after different diets. If analysis was carried out after perchloric acid pretreatment, as is commonly done, lower values were obtained than without such pretreatment. This could be explained by the existence of glycogen in forms of greater and lesser solubility. Evidence for this was obtained by quantitative analysis of glycogen in the perchloric acid extract. The acid soluble form in the extract constituted a greater fraction of total glycogen at supra-normal than at low concentrations. This might also explain why supra-normal levels of glycogen cannot be detected by histochemical methods. The existence of soluble and insoluble glycogen may also be of physiological importance, e.g. by interacting differently with enzymes of glycogen metabolism.