Glycogen phosphorylase kinases in soluble fractions of various rat tissues were examined for the pH 6.8/8.5 activity ratio, Ca2+-dependency, activation by cyclic AMP-dependent protein kinase (protein kinase A), and reactivity with anti-skeletal muscle phosphorylase kinase serum. The enzymes could be divided into at least two major groups; muscle and liver types. The muscle type, that has a low value of pH 6.8/8.5 activity ratio, is highly dependent on Ca2+, markedly activated by protein kinase A, and strongly inhibited by the antiserum. Inversely, the liver type, that has a high value of pH 6.8/8.5 activity ratio, is poorly dependent on Ca2+, not activated by protein kinase A, and weakly inhibited by the antiserum. The enzymes from heart and skeletal muscle were similar and belonged to the former entity. Whereas, the enzymes from liver, kidney, spleen, lung, and testis appeared to belong to the latter entity. The enzyme from brain apparently differs from these entities, and seems to be an intermediate type or a hybrid of the two.