Electro-stimulation alters muscle metabolism and the extent of this change depends on application intensity and duration. The effect of 14 days of chronic electro-stimulation on glycogen turnover and on the regulation of glycogen synthase in fast-twitch muscle was studied. The results showed that macro- and proglycogen degrade simultaneously during the first hour of stimulation. After 3 h, the muscle showed net synthesis, with an increase in the proglycogen fraction. The glycogen content peaked after 4 days of stimulation, macroglycogen being the predominant fraction at that time. Glycogen synthase was determined during electro-stimulation. The activity of this enzyme was measured at low UDPG concentration with either high or low Glu-6-P content. Western blots were performed against glycogen synthase over a range of stimulation periods. Activation of this enzyme was maximum before the net synthesis of glycogen, partial during net synthesis, and low during late synthesis. These observations suggest that the more active, dephosphorylated and very low phosphorylated forms of glycogen synthase may participate in the first steps of glycogen resynthesis before net synthesis is observed, while partially phosphorylated forms are most active during glycogen elongation.