Rat tissue levels of Ca 2+· calmodulin-dependent protein kinase II (protein kinase II) and Ca 2+· phospholipid-dependent protein kinase (protein kinase C) were selectively assayed using the synthetic peptide syntide-2 as substrate. The sequence of syntide-2 (pro leu ala arg thr leu ser val ala gly leu pro gly lys lys) is homologous to phosphorylation site 2 in glycogen synthase. The relative V max K m ratios of the known Ca 2+-dependent protein kinases for syntide-2 were determined to be as follows: protein kinase II, 100; protein kinase C, 22; phosphorylase kinase, 2; myosin light chain kinase, 0.005. Levels of protein kinase II were highest in cerebrum (3.36 units/g tissue) and spleen (0.85 units/g) and lowest in testis (0.05 units/g) and kidney (0.04 units/g). Protein kinase II activity was localized predominantly in the 100,000 g particulate fraction of cerebrum and testis, in the supernatant fraction of heart, liver, adrenal, and kidney, and about equally distributed between particulate and supernatant in spleen and lung. Likewise, protein kinase C activity was highest in cerebrum (0.56 units/g) and spleen (0.47 units/g), and the majority of activity was present in the cytosolic fraction for all tissues measured except for cerebrum and testis in which the kinase activity was equal in both fractions. Finally, the ratios of protein kinase II to protein kinase C were different in various rat tissues and between particulate and supernatant fractions. These results suggest somewhat different functions for these two Ca 2+-regulated, multifunctional protein kinases.