Effect of galactosamine on glycogenolysis was studied in isolated hepatocytes. It was found that addition of galactosamine strongly inhibited glycogenolysis in normal hepatocytes. Galactosamine-inhibited glycogenolysis was not stimulated by epinephrine or glucagon. This inhibition was specific as no such inhibition was observed with galactose, 2-deoxy-glucose or glucosamine. The glucagon-stimulated cyclic AMP formation in galactosamine-treated hepatocytes was the same as in normal cells; Glc-1-P and Glc-6-P did not accumulate nor was lactate formation enhanced. The glucose production by hepatocytes from regenerating liver was only slightly inhibited by galactosamine and glucagon addition stimulated glycogenolysis in the presence of the amino sugar.