The mechanism of activation of glycogen phosphorylase is incompletely understood, although adenosine and inosine nucleotides are known to be important allosteric activators. In this study the activation of glycogen phosphorylases a and b from bovine liver by adenosine 5'-monophosphate (AMP) and inosine 5'-monophosphate (IMP) has been investigated and the results compared with the activation of the muscle isozyme by the same nucleotides. Enzyme activity was determined by spectrophotometric measurement of inorganic phosphate produced in the phosphorylase-catalysed reaction of glycogen synthesis. Liver phosphorylase b binds both nucleotides non-co-operatively (Hill coefficients of 1.0 +/- 0.1), with changes in the maximum velocity to 75 or 80 mumol min-1 mg-1 in the presence of adenosine 5'-monophosphate or inosine 5'-monophosphate, respectively, but no change in the enzyme affinity towards the substrate, glucose-1-phosphate. Binding of glucose-1-phosphate is co-operative and the kinetic data have been fitted with the Monod-Wyman-Changeux model. Liver phosphorylase a has a maximum velocity similar to that of the b form in the presence of nucleotides. Binding of glucose-1-phosphate to the enzyme is non-co-operative (Hill coefficient of 1.0 +/- 0.1) and the affinities in the presence of the nucleotides (Michaelis constants of 28 +/- 0.2 mM or 27 +/- 0.2 mM for adenosine 5'-monophosphate or inosine 5'-monophosphate) are stronger than those of the b form. It is concluded that the activity of bovine liver phosphorylase a and b is similarly influenced by adenosine 5'-monophosphate or inosine 5'-monophosphate. The b form seems to behave like muscle phosphorylase b in response to inosine 5'-phosphate; however, the binding of adenosine 5'-phosphate does not induce the conformational change necessary to activate the liver enzyme, as occurs with the muscle isozyme.