Since protein kinase C (PKC) is crucial in a number of receptor mediated processes for protein phosphorylation, we examined changes of PKC (PKC-$\alpha ,$ -$\beta ,$ -$\varepsilon$ and -$\zeta$ isozymes) in diabetic hearts. In this study, rats were made diabetic by an intravenous injection of streptozotocin (65 mg/kg body wt) and the hearts were removed 8 weeks later. At 6 weeks, some of the diabetic animals received subcutaneous injection of insulin (3U/day) for 2 weeks. PKC activity was assayed by measuring the transfer of $\sp{32}$P from $\lbrack\gamma$-$\sp{32}$P) ATP to a synthetic substrate. Relative protein content of the specific PKC isozymes was obtained by using immuoblot analysis in cytosolic, particulate and homogenate fractions from control, diabetic and insulin-treated diabetic rat hearts. In another series of experiments, genetic cardiomyopathic hamsters (250-300 day old; UM-X 7.1) were employed to test if the observed changes in diabetic hearts are similar to other types of cardiomyopathy. In order to determine whether changes in the PKC of cardiomyopathic hearts are specific in nature, we a so examined PKC in skeletal muscle as well as protein kinase A (PKA) in both cardiac and skeletal muscles. (Abstract shortened by UMI.)