The glucose analogue 1-deoxynojirimycin (dNOJ) and some of its N-substituted derivatives have recently been described as potent inhibitors of the hepatic glycogenolysis induced by glucagon, Ca2+ ionophores or anoxia. The inhibition increased with time, in spite of a persistently high level of phosphorylase a [Bollen, M., Vandebroeck, A. & Stalmans, W. (1988) Biochem. Pharmacol. 37, 905-909]. dNOJ equilibrates within 1 min across the plasma membrane of hepatocytes. It is not phosphorylated or oxidized in the cell. The observation that dNOJ did not affect gluconeogenesis excludes the possibility that glucose-6-phosphatase is the target for the inhibition of glucose production from glycogen. Neither were the catalytic activities of phosphoglucomutase and phosphorylase a affected by the compound. dNOJ and two N-substituted derivatives inhibited instantaneously and completely the alpha-1,6-glucosidase activity of the debranching enzyme, with I50 values in the mumolar range. In contrast, the glucanotransferase activity of the latter enzyme was not inhibited by the compounds at 0.2 mM. The effect of dNOJ was further studied in an in vitro model system of glycogenolysis. The results were compatible with a block of glycogenolysis at the time when phosphorylase has removed the available glucosyl residues from the outer chains of the glycogen particles. This mechanism appears to account for the lag in the response of glycogenolysis to dNOJ.