This study was aimed at the evaluation of aortic impedance in patients with congestive heart failure. Aortic impedance (simultaneous measurements of aortic pressure and blood flow), mean (Wm) and pulsatile (Wp) powers were compared in 11 normal subjects and in 12 patients with heart failure. Pulse wave velocity (C: modified Moëns-Korteweg equation, simultaneous measurements of aortic pressure and radius) was determined under control conditions in all normal subjects and in 7 patients with heart failure. Impedance curves in patients with heart failure were characterized by increased values of the impedance modulus at 0 Hz (peripheral resistance) and at low frequencies. The characteristic impedance, C, and phase were not different from normal subjects. In six patients with heart failure, impedance curves were studied during nitroprusside infusion. During the infusion of the vasodilator, the impedance modulus at 0 Hz and at low frequencies decreased. The characteristic impedance was unchanged. The zero intercept of the phase was shifted towards lower frequencies. These results show that the changes in impedance curves in patients with heart failure are due to greater peripheral resistance and wave reflection. During nitroprusside infusion the stroke volume increased and the aortic blood flow became more pulsatile (greater values of low frequency components). This modification accounts for the increased values of Wm and Wp, and is related to decreased peripheral resistance and wave reflection.