In end-stage heart failure due to idiopathic dilated cardiomyopathy ₁-adrenoceptors are downregulated and G₁-proteins are upregulated. The aim of the present study was to investigate the influence of the angiotensin-converting enzyme inhibitor captopril on -adrenoceptor density and G_i-proteins in sequential endomyocardial biopsies. Nineteen patients with mild to moderate congestive heart failure due to idiopathic dilated cardiomyopathy (NYHA Class II–III) were studied before and after 8–11 weeks of therapy. Patients were randomised into a captopril and a control group; 9 patients received captopril 12.5–50 mg per day, (divided in 2–3 doses) p.o. in addition to conventional therapy with digoxin and diuretics, and 10 controls received conventional therapy only. Echocardiography, spiroergometry, right heart catheterisation and endomyocardial biopsies were performed before (baseline) and after treatment. Compared to baseline, captopril increased total -adrenoceptor density by selectively increasing ₁-adrenoceptors (31.6 vs 41.2 fmol·mg^–1; p<0.05) but="" had="" no="" significant="" effect="" on="">_i-proteins. The results indicate that treatment with angiotensin-converting enzyme inhibitors partly restores myocardial ₁-adrenoceptor density, and this action effect may contribute to the clinical improvement of patients with idiopathic dilated cardiomyopathy treated in this way.