Biomechanical indices of left ventricular performance were measured and computed by an on-line minicomputer in 13 isolated perfused working rat hearts subjected to a regimen of left atrial filling pressures and heart rates, to determine perfusion conditions for optimum heart work. The authors found that maximum values of most of the indices are achieved at a left atrial filling pressure of 20 to 25 cm H₂O and a heart rate of 330 to 360 beats/min. Most of the performance indices when considered as functions of left atrial filling pressure display Starling-type behaviour: they rise with increasing left atrial pressure, reach a maximum at 20 to 25 cm H₂O, and decline with further increases in left atrial pressure. Values for left ventricular (dP/dt)_max, between 6000 to 8000 mm Hg s^-1, and -(dP/dt)_min, between 4000 to 6000 mm Hg s^-1, are considerably higher than values previously reported. The authors believe this to be due to improved sampling techniques.