Type 12 interneurons in pedal ganglia of Clione limacina exerted a strong influence upon the locomotor generator during "intense" swimming. These neurons generated "plateau" potentials, i.e. their membrane potential had two stable states: the "upper" one when a neuron was depolarized, and the "down" one, separated by 30-40 mV. The interneurons could remain in each state for a long time. Short depolarizing and hyperpolarizing current pulses, as well as excitatory and inhibitory postsynaptic potentials, could transfer the interneurons from one state to another. When the pedal ganglia generated the locomotory rhythm, type 12 neurons received an EPSP and passed to the "upper" state in the V2-phase of a locomotor cycle. They remained at this state until the beginning of the D1-phase when they received an IPSP and passed to the "down" state. The EPSP in type 12 neurons was produced by type 8d neurons, and the IPSP by type 7 neurons. Type 12 neurons exerted inhibitory influences upon many neurons active in the V1 and V2 phases, and excitatory influences upon the D-phase interneurons (type 7). The functional role of type 12 neurons was to limit the activity of neurons discharging in the V-phase of a locomotory cycle. In addition, they enhanced the excitation of the D-phase neurons and promoted, thus, the transition from the V-phase to the D-phase.