Abstract Tetanic contraction of rat skeletal muscle, previously stimulated to increase its glucose-6-P independent proportion of glycogen synthetase (I-GS), results in the transformation of GS from the independent to the dependent (D-GS) form. This I to D conversion is very rapid, since it takes place during the first 5 sec of stimulation, and occurs simultaneously with an increase of phosphorylase a . Possible mechanisms of this conversion have been investigated. During the ensueing recovery period glycogen phosphorylase is reverted to the less active b form, and GS to the more active I form. The simultaneous and opposite changes in the active forms of both enzymes are mantained, at least, through four cycles of contraction and recovery, suggesting the presence of a system which seems to interlook both enzymes to allow maximal efficiency during glycogenolysis or glycogenesis.