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. We 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. We believe this to be due to improved sampling techniques.