1. In bipolar neurons of the P.D. organ of Callinectes sapidus the initiation of impulses at the soma requires depolarization of at least 4-5 mV.

2. Impulses produced by mechanical stimulation of the organ arise abruptly from the resting (potential) level without prepotential, and it is impossible to demonstrate the occurrence of generator potentials of the magnitude required for spike triggering at or near the soma.

3. Anti- and orthodromic invasion of the soma may be differentially blocked by hyperpolarization; orthodromic block often reveals a small potential that appears to result from impulse activity in the distal process of the cell.

4. Blockade of the soma spike to leave only the electrotonic potential of the distal process results in interruption of axonal transmission in the impaled cell.

5. It is concluded that P.D. bipolar cells originate impulses in their distal processes with subsequent conduction through the soma into the axon.

This research was supported in part by Grant no N.B.-04588 from the National Institute of Neurological Diseases and Blindness, U.S. Public Health Service.