1. The duration and frequency of tentacle-regulating potentials (TRPs) recorded from the vacuole of Noctiluca miliaris were modified by both steady and abrupt changes in the vacuolar potential. The spike frequency increased linearly with an increase in the vacuolar negativity over the range of 0 to -50 mV.

2. A transient decrease of impedance across the soma occurs coincidentally with the negative-going TR spike recorded from the vacuole. Local measurements of impedance changes as well as local surface potential recordings indicate that the TR spike does not propagate over the remainder of the cell from its region of origin near the base of the tentacle. Specimens without visible tentacles exhibited TRPs of normal shape.

3. Concurrent potential recordings from the vacuole and the perinuclear cytoplasm indicate that the vacuolar membrane bordering that portion of the cell is inactive. Except for a relatively more pronounced post-spike hyperpolarization the TRP shape and polarity in the perinuclear cytoplasm, referred to the sea-water bath, are similar to those recorded from the vacuole, but are of smaller amplitude.

4. The TRP appears to consist of a pre-spike pacemaker potential and an action potential having a prolonged post-spike plateau, the pattern recorded from the vacuole being the approximate inverse of that which would be recorded if a microelectrode could be introduced into the cytoplasm adjacent to the electrically active portion of membrane. Several of the findings indicate that the active membrane faces the vacuole and is electrotonically isolated from the perinuclear cytoplasm except through the vacuole.

On leave of absence from Biological Institute, Faculty of Science, Tohoku University, Sendai, Japan.