The authors evolved an automatic system for the measurement and analysis of the course of pressure in the left ventricle of the isolated laboratory rat heart perfused in vitro after Langendorff. Attached to it is a microcomputer which automatically samples the course of the left ventricular pressure curve over a segment comprising five cardiac cycles and, within 30 s, evaluates the mean systolic and diastolic pressure, the maximum rate of the increase and decrease in pressure, the contractility index, the mean integral pressure and the heart rate. The apparatus shows a standard error of less than 1% for pressure and of up to 2% for derivation.