Calcium concentration changes during oscillations of the membrane potential of crab (Cancer irroratus or Cancer borealis) stomatogastric neurons were monitored at many positions by using the calcium indicator dye arsenazo III and a photodiode array. Data analysis algorithms using signal averaging techniques were developed to improve the time resolution of the measured calcium changes. As previously reported, calcium oscillations were detected from all regions of the neuropil but not from the soma or axon. In some cells step increases in intracellular neuropil calcium were correlated with each of the action potentials in the burst (on the peak of the voltage oscillation). In other cells we observed calcium oscillations phase-locked to the membrane potential with no spike-related component. A few cells had both spike-evoked and graded potential components to the calcium oscillations. In those cells, the spatial distribution of the spike-correlated calcium influx differed from that of the voltage-oscillation-correlated calcium influx, suggesting that different neurites might interact with their postsynaptic targets with different mixtures of graded and spike-correlated transmitter release.