The activities of glycogen synthase and glycogen phosphorylase were quantitated in liver and skeletal muscle removed following glucose infusion in hemodynamically stable endotoxin-treated rats. Four hours after the IV injection of endotoxin or saline, rats were infused with 235 mumole/min/kg of glucose or saline for up to 4 additional hr. Saline-infused endotoxemic rats had lower basal glycogen content in muscle and liver, which was associated with an increased phosphorylase a activity in both tissues compared to controls. During the glucose infusion, the rate of glycogen repletion in muscle was similar in the two groups. Skeletal muscle phosphorylase a and glycogen synthase I & D activities were elevated above control values in endotoxemia, while glycogen synthase I activity remain unchanged. These changes in the activity of muscle phosphorylase and synthase are consistent with an increased flux of carbon into and out of glycogen and a normal rate of net glycogen synthesis during glucose infusion in endotoxin-treated rats. In contrast to muscle, hepatic glycogen synthesis by endotoxemic animals was reduced compared to glucose-infused controls. Hepatic glycogen repletion in control animals appeared to be mediated primarily by a glucose-induced suppression of phosphorylase a activity rather than an increased glycogen synthase activity. Glucose infusion failed to decrease phosphorylase a activity in endotoxin-treated rats, which may be causally related to the impaired ability of these animals to replete liver glycogen.