To determine the mechanical properties of the left ventricle as a pump, a new technique was developed based on impulse-response concepts. In 11 isolated canine hearts contracting isovolumetrically, steplike changes in ventricular volume (.DELTA.V = 1-3 ml) were produced by flow pulses lasting 35-50 ms; such steps could be applied at any phase of systole. The resultant change in ventricular pressure (.DELTA.P) between the perturbed beat and the preceding undisturbed beat was examined during the time of the pulse and the remainder of contraction. The ratio .DELTA.P/.DELTA.V [ratio of experimental and control pressure difference to total change in volume], a measure of ventricular viscoelasticity, varied throughout systole in characteristic patterns. Following a volume step early in systole, .DELTA.P/.DELTA.V increased steadily during contraction (maximum values, 3.6-11 mmHg/ml). A volume step imposed late in systole produced a bimodal .DELTA.P/.DELTA.V with the 1st transient peak in phase with the flow pulse. Analyzing these patterns revealed that the mechanical components contributed differently to .DELTA.P/.DELTA.V depending on the time of the flow pulse: for early pulses, elasticity dominated; for late pulses, a resistive component was more evident. Using pulse-response measurements, the course of ventricular pressure during ejections having arbitrary flow contours was predicted. Measured and predicted pressures agreed within 15%. The flow-pulse technique is a comprehensive method for the study of ventricular mechanical properties.