The role of the angiotensin converting enzyme (ACE) in the regulation of local synthesis of angiotensin II has not been clearly defined. The authors investigated the local factors which might orientate the effects of ACE inhibitors to particular organs in the Wistar rat. The in vivo study of the effects of low doses of ramipril on the myocardium showed that cardiac ACE was significantly inhibited by the non-antihypertensive dose of 0.01 mg/kg whereas the inhibition only occurred from doses higher than 0.1 mg/kg in the other tissues studied. In the kidney: the affinity of 3H-ramiprilate for the brush borders of the proximal tubular cells was increased by high concentrations of chloride ions as observed in the renal parenchyma, the presence of esterases makes local activation of ramipril (diester) into ramiprilate (active diacid) possible, prolonged treatment with ramipril leads to a lowering of the concentration of ACE in the brush border of the proximal tubular cells, verified after the elimination of the ACE inhibitor fixed on the tissue. These data indicate that the myocardium and the kidney could be privileged targets of the action of ramipril.