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ACCO1RDING to our present state of know-ledge it seems very probable that the U wave of the electrocardiogram is caused by the summation of afterpotenitials in cardiac muscle, and that the normal positive U waves are caused by the summation of negative afterpotentials. Since these potentials are accompanied in nerve or muscle by a temporary reduction of the threshold of excitability (supernormal phase of excitability), it follows that the U wave must correspond to this supernormal phase, as was first suggested by Nahum and Hoff 4 who observed that coupled ventricular extrasystoles usually occur during the writing of the U wlave. In the present paper it was intended to test this concept by determining whether or not factors that, alter the amplitude or time of appearance of the U wave have a corresponding effect on the incidence or time of appearance of ventricular extrasystoles. Furthermore, it was of interest to know whether persons showsing primary U-wave inversion, which was attributed to the appearance of positive afterpoteintials (corresponding to a phase of subnormal excitability, show a different incidence or timing of extrasystoles than persons with normal U waves. One of the most important factors that influence the time of appearance of the 1U wave is the heart rate. Therefore it, was important to determine whether in the regularly beating heart the coupling interval of vrenitricular extrasystoles decreases with increasing heart rate

XI. Coupling Intervals of Ventricular Extrasystoles in Relation to the Heart Rate, the U Wave, and the Supernormal Phase of Excitability