The conformational transition of phosphorylase b which occurs upon pH change from 6.0 to 7.0 is linked to the pH-dependent activity regulation. Skeletal muscle dipeptides, carnosine and anserine, activate rabbit and bovine phosphorylase b at pH 6.5-6.0 and inhibit it at pH 6.5-7.0. The enzyme activation constant was found to be equal to 20 mM, and the inhibition constant corresponded to 22 mM for both carnosine and anserine. The data obtained suggest that glycogen phosphorylase b adopts different conformational states at pH 6.0 and 7.0. Evidence for the pH-induced conformational transition of the enzyme was obtained using chemical modification of histidine residues. The change in the phosphorylase activity under effects of the dipeptides in the pH range of 7.0-6.0 may be due to their physiological role in muscle contraction.