Phosphoinositide turnover and protein kinase C (PKC) mediate the signaling of angiotensin II, which plays a pivotal role in ventricular remodeling after myocardial infarction (MI). To determine whether PKC is activated after MI, rat hearts after MI were subjected to in vitro quantitative autoradiography with [3H]phorbol 12,13-dibutyrate (PDBu), which is highly selective for PKC. [3H]PDBu binding in the infarcted area increased significantly compared with the non-infarcted region 7 and 21 days after MI, but not 1 and 3 days and 10 months after MI. [3H]PDBu binding in the noninfarcted area was similar to that in the sham-operated rats. Immunohistochemical analysis revealed that abundant macrophages (7 days after MI), fibroblasts, and myofibroblasts (7 and 21 days after MI) occupied the infarcted region. To investigate whether myocardial [3H]PDBu binding is affected by captopril, hearts were subjected to in vitro autoradiography with [3H]PDBu after 1- or 3-week captopril treatment or no treatment. Captopril treatment significantly suppressed [3H]PDBu binding in the infarcted area 3 weeks after MI, but not 1 week after MI nor in the noninfarcted areas. These results suggest that PKC is upregulated during the healing and fibrogenic process after MI and that captopril treatment suppresses the upregulation in the infarcted area.