The activities of cardiac protein kinase C (PKC) were examined in hemodynamically assessed rats subsequent to myocardial infarction (MI). Both Ca²⁺-dependent and Ca²⁺-independent PKC activities increased significantly in left ventricular (LV) and right ventricular (RV) homogenates at 1, 2, 4, and 8 wk after MI was induced. PKC activities were also increased in both LV and RV cytosolic and particulate fractions from 8-wk infarcted rats. The relative protein contents of PKC-α, -β, -ε, and -ζ isozymes were significantly increased in LV homogenate, cytosolic (except PKC-α), and particulate fractions from the failing rats. On the other hand, the protein contents of PKC-α, -β, and -ε isozymes, unlike the PKC-ζ isozyme, were increased in RV homogenate and cytosolic fractions, whereas the RV particulate fraction showed an increase in the PKC-α isozyme only. These changes in the LV and RV PKC activities and protein contents in the 8-wk infarcted animals were partially corrected by treatment with the angiotensin-converting enzyme inhibitor imidapril. No changes in protein kinase A activity and its protein content were seen in the 8-wk infarcted hearts. The results suggest that the increased PKC activity in cardiac dysfunction due to MI may be associated with an increase in the expression of PKC-α, -β, and -ε isozymes, and the improvement of heart function in the infarcted animals by imidapril may be due to partial prevention of changes in PKC activity and isozyme contents.