1. The site and mechanism of initiation of the rhythmic action potentials controlling the somatic musculature of Ascaris have been reinvestigated.
2. Polarization of the muscle syncytium by direct current injection revealed little accommodation. Action potentials are generated continuously at this region at a frequency which depends on the membrane potential.
3. Excitatory and inhibitory nerve fibres control the membrane potential of the syncytial membrane and, therefore, the frequency of spike firing. The effects of stimulation of these fibres are described.
4. The resumption of electrical activity when cooled, quiescent preparations were warmed up was studied. The first signs of activity are slow rhythmic depolarizations on which bursts of abortive spikes are superimposed. When the amplitude of the transients in each burst increases sufficiently they unite into a large, single action potential.
5. Evidence is presented suggesting that each of the abortive spikes represents the separate, subthreshold excitation of one of the terminal branches of the muscle arm, due to a low safety margin for the conduction of impulses towards the muscle belly.
6. Small (1-2 mV.) spontaneous, apparently random, depolarizations and hyperpolarizations have been recorded with microelectrodes inserted into the syncytial region. Their possible synaptic origin is discussed.