Rhythmic bursting in neurons is accompanied by dynamic changes in intracellular Ca²⁺ concentration. These Ca²⁺ signals may be caused by membrane potential changes during bursting and/or by synaptic inputs. We determined that membrane potential is responsible for most, if not all, of the cytoplasmic Ca²⁺ signal recorded during rhythmic bursting in two neurons of the escape swim central pattern generator (CPG) of the mollusc, Tritonia diomedea: Ventral Swim Interneuron B (VSI) and Cerebral neuron 2 (C2). Ca²⁺ signals were imaged with a confocal laser scanning microscope while the membrane potential was recorded at the soma. During the swim motor pattern (SMP), Ca²⁺ signals in both neurons transiently increased during each burst of action potentials, with a more rapid decay in secondary than in primary neurites. VSI and C2 were then voltage-clamped at the soma and each neuron's own membrane potential waveform recorded during the SMP was played back as the voltage command. In all regions of VSI, this completely reproduced the amplitude and time course of Ca²⁺ signals observed during the SMP, but in C2 the amplitude was lower in the playback experiments than during the SMP. Therefore in VSI, the cytoplasmic Ca²⁺ signal during the SMP can be accounted for by its membrane potential excursions, whereas in C2 the membrane potential excursions can account for most of the SMP Ca²⁺ signal.