Angiotensin converting enzyme (ACE) is a key factor in the regulation of two peptide systems: the renin angiotensin system (RAS) and the kinin-kallikrein system (KKS). Since it is involved in the biosynthesis of Angiotensin II (Ang II) as well as in the degradation of bradykinin (BK) it could play an important role in cardiovascular physiology and pathophysiology. ACE is widely expressed in the heart and upregulated in pathophysiological situations such as heart failure and cardiac hypertrophy. In addition, inhibition of ACE has beneficial effects in these conditions. Whereas the regulation of cardiac ACE has been studied extensively, little is known concerning the cellular expression of ACE in cardiac tissue. To define the cellular localization of ACE mRNA expression in the rat heart, we separated coronary microvascular endothelial cells from cardiac myocytes using differential centrifugation and growth on selective media. ACE mRNA expression was measured by a specific polymerase chain reaction assay after reverse transcription (RT-PCR) in different cardiac cells. The studies showed that ACE is differentially expressed in endothelial cells as well as in cardiac myocytes. This differential regulation of ACE in myocytes and non-myocytes may play a role for the diverse actions of the cardiac angiotensin system under physiological and pathological conditions.