A method is described for the continuous recording of changes in the length of a segment of left ventricular myocardium, and its advantages and limitations are discussed. The curve depicting the relation between ventricular diastolic pressure and simultaneous changes in the length of a myocardial segment is presented. For a given increment in pressure, the myocardial segment length increases more at low ventricular diastolic pressure than at high pressure. Atrial systole causes a substantial increase in myocardial segment length when the ventricle is on the sensitive part of its pressure-length curve.