Apart from the inversion of the P-wave met with in Lead III in the human electrocardiogram in the presence of an ectopic, heterogenetic rhythm,1 associated with a definite change in the location of the pacemaker, and as a result of disturbances in the respiratory rhythm, such negativity of the atrial deflection is but occasionally seen, though it has seemed to us that the frequency of its incidence in this derivation in pathologic hearts is greater than one is led to expect from a review of the literature. That Lead III shows the greatest susceptibility to these changes of form and direction is a fact of common observation, and the frequent inversion of the T-wave seen in this lead in otherwise normal galvanometric curves is additional evidence of the variation in electrical negativity met with in clinical observations. It has been shown by Einthoven, Fahr and de Waart2 that the P-deflection