To analyze cardiac motion during ventricular fibrillation (VF), we used transesophageal echocardiography to study nine male subjects, aged 44 +/- 7 years, affected by heart disease who have poor left ventricular function, during implantation of an Implantable Cardioverter Defibrillator, when VF is induced several times to determine the defibrillation threshold. Wall and valvular motion, transmitral and transaortic blood flow, and blood echoreflectivity were evaluated in all patients. Moreover, in basal conditions, during VF, 1 and 5 minutes after restoration of basal rhythm, we calculated the left ventricular end-diastolic volume (EDLVV) and area (EDLVA), the left ventricular end-systolic volume and area, the ejection fraction, and the fractional area change with a four-chamber echocardiographic view. At the onset of VF, the myocardium and valves exhibited a chaotic motion. About 10 seconds later the oscillatory movement of the heart walls became more ample and regular; the mitral valve showed a cyclic closure and opening with a forward flow, and the aortic valve exhibited similar behavior, although at a lower intensity. A spontaneous echo contrast appeared inside the atrial and ventricular cavities, gradually becoming an incrt homogeneous mass that was completely flushed away with the restoration of the basal rhythm. When VF started, EDLVV (286 +/- 98 ml) and EDLVA (50 +/- 16.5 cm2) decreased abruptly (EDLVV = 182 +/- 65 ml, p < 0.02; EDLVA = 38 +/- 9.2 cm2,p < 0.05); so did ejection fraction (31.8% +/- 15% versus 11% +/- 5%; p < 0.003) and fractional area change (25.8% +/- 6.5% versus 7% +/- 3.4%; p < 0.001). When the basal rhythm was restored, the heart extended again and EDLVV, EDLVA, ejection fraction, and fractional area change after 1 and 5 minutes were similar to those calculated before induction of VF. This behavior was observed during both the first and last induced VF. Thus during VF, great variations of heart morphology and dynamics, as well as blood echoreflectivity, occur; the heart seems to make attempts to organize its dynamics during the arrhythmia. Repeated episodes of VF and defibrillation with low energies do not seem to worsen left ventricular dynamics even in impaired hearts.