Kinetic studies of both liver and muscle phosphorylase a demonstrate that caffeine and glucose inhibit the binding of glucose 1-phosphate to the enzyme in a synergistic competitive and nonexclusive manner. Inhibition studies for numerous other caffeine analogs show that the muscle enzyme has a relaxed specificity for this negative effector. The liver enzyme is more discriminating by preferential binding of methylated oxypurines. Physiological concentrations of AMP and ATP, which affect the enzymic activity at a separate site, prevent glucose from effectively inhibiting the enzyme. The addition of the second synergistic ligand improves the binding of glucose. These data suggest that glucose homeostasis as regulated by phosphorylase may be dependent on a second ligand and that the role of glucose in this physiological process may have been overestimated. A structural rationalization of this synergistic response is discussed with reference to the crystal structure of the muscle enzyme.