By means of a DC bridge circuit one microelectrode was used for simultaneously passing current and recording transmembrane potentials. In some cells, depolarization increased the frequency of discharge whereas hyperpolarization decreased the frequency; the frequency/current relation was sigmoid. In other cells, polarizing currents were without effect upon frequency. The change in action potential magnitude was in proportion to the degree of polarization. From control values of about 5 mv/sec., the slope of the pacemaker potential increased to 60 mv/sec. upon depolarization and diminished to zero upon hyperpolarization. In many cells a transient hyperpolarization was produced on the cessation of depolarizing currents. The voltage/current relationship was linear and had a slope of about 13 MΩ. With an AC bridge circuit, the cell capacitance averaged 800 pf and the time constant, 9.6 msec. Rm was estimated to be 480 Ω-cm2 and Cm, 20 µf/cm2. The magnitudes of some prepotentials were affected by polarizing currents, which suggests that the prepotentials represent postsynaptic potentials.

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