Bioelectric activity in single cells of the moth myocardium has been measured in situ with intracellular microelectrodes with particular reference to the bizarre ionic medium which bathes the tissues. Resting potentials averaged 47 mv, inside negative with respect to outside, despite a value of 11 mv calculated on the basis of transmembrane potassium concentration gradients. Action potentials overshoot as much as 12 mv in the virtual absence of extracellular sodium. Two "types" of action potentials have been recorded; one that resembles vertebrate atrial action potentials is found in the cephalic region of the tubular heart, and the other, similar in contour to vertebrate ventricular action potentials, is found in the areas posterior to the first abdominal segment. Histological sections indicate no structural differences between the two areas. Typical cardiac pacemaker type potentials occur but are not topographically localized. The effects of the omission from the perfusion fluid of the four major cationic constituents, Na+, K+, Ca++ and Mg++, on resting and action potentials may be summarized as: no effect, hyperpolarization, prolonged repolarization, and depolarization, respectively.

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