Glycogen phosphorylase in the liver, muscle, and brain initiate glycogenolysis by releasing glucose-1-phosphate from glycogen. Glycogen phosphorylase inhibitor is a cell-permeable acyl urea first identified as an inhibitor of human liver glycogen phosphorylase (IC50 = 53 nM). It blocks glucagon-induced hepatic glycogenolysis in vivo. Glycogen phosphorylase inhibitor has been used to study glycogen utilization in human liver HepG2 cells, retinal explants, and human T lymphocyte Kit 225 cells.