Explants of fetal rat liver were maintained in organ culture. After 40 hr of incubation in control medium, insulin causes a net synthesis of glycogen. The action of insulin is augmented by a high glucose concentration (50 mm) and is partially blocked by cycloheximide but not by actinomycin D. The addition of epinephrine causes glycogenolysis. The isolated explant of fetal rat liver appears to be an excellent in vitro model system for studying hormonal regulation of hepatic glycogen metabolism.(Endocrinology92: 584, 1973)