Calcium dynamics in leech neurons were studied using a fast CCD camera. Fluorescence changes (F/F) of the membrane impermeable calcium indicator Oregon Green were measured. The dye was pressure injected into the soma of neurons under investigation. F/F caused by a single action potential (AP) in mechanosensory neurons had approximately the same amplitude and time course in the soma and in distal processes. By contrast, in other neurons such as the Anterior Pagoda neuron, the Annulus Erector motoneuron, the L motoneuron and other motoneurons, APs evoked by passing depolarizing current in the soma produced much larger fluorescence changes in distal processes than in the soma. When APs were evoked by stimulating one distal axon through the root, F/F was large in all distal processes, but very small in the soma. Our results show a clear compartmentalization of calcium dynamics in most leech neurons, in which the soma does not give propagating action potentials. In such cells the soma, while not excitable, can affect information processing by modulating the sites of origin and conduction of AP propagation in distal excitable processes.