Nine patients with primary, non-obstructive myocardiopathy were treated during 6 months with captopril in daily doses of 75 to 225 mg. Early signs of haemodynamic improvement included significant increase in cardiac output (p Less Than 0.01) and stroke volume (p Less Than 0.01) and significant decrease in peripheral resistance (p Less Than 0.001) and pulmonary wedge pressure (p Less Than 0.001). These effects were sustained throughout treatment and were accompanied with noticeable improvement in the patients' functional status. Echocardiographic studies showed a reduction in the telesystolic (p Less Than 0.01) and telediastolic (p Less Than 0.05) diameters of the left ventricle after 6 months's treatment. The preejection period, calculated from phonomecanograms, was also significantly reduced. Plasma renin activity initially rose (p Less Than 0.01), then fell below pre-treatment values (p Less Than 0.02). Plasma aldosterone levels decreased (p Less Than 0.05), together with angiotensin-converting enzyme levels (p Less Than 0.05). Basal plasma renin activity correlated with both basal peripheral resistance (r = 0.72; p Less Than 0.02) and fall in peripheral resistance (r = 0.89; p Less Than 0.01). There was no tachyphylaxis nor evidence of blood, kidney or liver toxicity.