ExtractGlycogen metabolism was studied in the isolated perfused liver of the monkey conceptus at 90% of gestation using anin siturecirculating perfusion system. Net uptake of glucose and galactose and the activities of the enzymes, glycogen synthetase and phosphorylase, were studied in response to varied perfusate composition. Synthetase activity was expressed as %1, the percentage of total synthetase activity in the active form. Perfusate glucose concentrations as high as 700 mg/100 ml did not lead to net glucose uptake or to an increase in the baseline %I synthetase (4 ± 1, mean ± SEM). In the presence of 300 mg/100 ml glucose, insulin at 10-7M increased %I to 8 ± 2, and galactose > 75 mg/100 ml increased %I to 8 ± 1. The combination of galactose, glucose, and insulin increased %I to 40 ± 5. With this latter combination, synthetase activity was proportional to perfusale glucose concentration above 100 mg/100 ml. Phosphorylase activity was diminished by either galactose or insulin, and phosphorylase activity was lowest in the group receiving galactose, glucose, and insulin. Galactose was taken up by all livers, but net glucose uptake was not observed under any condition; net hexose uptake was observed in perfusions with galactose. Glycogen levels did not vary significantly with varied perfusate composition during the 30-min perfusion periods.SpeculationWe speculate that galactose may be uniquely important for neonatal liver glycogen synthesis, and that its action is mediated through reciprocal changes in the activities of the enzymes, glycogen synthetase and phosphorylase. If liver glycogen is important for acute neonatal glucose homeostasis, then galactose may also be essential for maintaining circulating glucose concentration by ensuring glycogen synthesis during feeding.