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A U T H O R ’ S SYNOPSIS The TMP was measured in normal parts of the left ventricular wall of two artificially perfused human hearts at different heart rates, using microelectrodes and a field effect transistor. The membrane resting potentials were-82 and-87 mV. The duration of the upstroke coinciding with the intrinsic deflection of the unipolar complex, recorded just before entering the cell, was 1 msec. No overshoot was present in one heart, because of cancellation of a possible over-shoot by the S-wave of the unipolar complex. The relation between frequency and duration of the TMP is identical with the QT-time-frequency relation, as found in extremity leads. The changes in duration of the TMP caused by changes in heart rate, result from changes in the duration of the plateauphase exclusively. The phase 3 of the TMP has a constant form, independent of rate and rhythm. The form of the T wave from driven hearts in clinically used ECG leads is not dependent on the stimulation rate. The advantages of the field effect transistor in intracellular recording are discussed. After the clinical death of a patient we had the opportunity to perfuse the isolated heart and to record the transmembrane potentials (TMP). The influence of heart rate on the form and duration of the TMP was studied. Methods The isolated human heart was perfused in a Langen-dorff system, described in detail elsewhere (Durrer,
G. E. FREUD (Sat,) studied this question.