In order to evaluate the effect of modifications in reflections timing on the ventricular performance, 10 isolated rabbit hearts were connected to rubber tubes of different lengths (0.5, 0.8, 1.0, 2.5 m) with terminal stopper and 2 cylindric small tubes as constant hydraulic resistance. The terminal connection produced wide reflections which returned to the ventricle in early, mid, late systole and in the diastolic phase (2.5 m). Ventricular and aortic pressure, and aortic flow were continuously recorded, sampled and stored on a personal computer. Instantaneous and total systolic ventricular work and stroke volume were calculated using an automatic procedure. Mean coronary perfusion pressure was calculated as difference between aortic and ventricular pressure in diastolic phase. The results demonstrate an important modification of the ventricular performance related to the anticipation of the reflected pulse. The anticipation produces a decrease in the stroke volume (-14.5%), a decrease in the external work of the ventricle (-21%) and a decrease in mean coronary perfusion pressure (-28%). The first two parameters describe the performance of the pump with respect to the load, and the third evaluates the perfusion of the pump itself. Reported to the clinical situation, these results demonstrate that an increased stiffness of the aorta has a double negative effect, a decrease in cardiac output and in external ventricular work, and moreover a decrease in coronary pressure, which produces a reduction of coronary flow.