The computation of motion plays a central role in visual orientation. The fly has been successfully used as a model system for analyzing the mechanisms underlying motion detection. Thereby, much attention has been paid to a neuronal circuit of individually identifiable neurons in the third visual ganglion that extracts different types of retinal motion patterns and converts these patterns into specific components of visual orientation behavior. The extended dendritic trees of these large cells are the sites of convergence of numerous spatially distributed local motion-sensitive elements. As is revealed by in vivo microfluorometric imaging, these cells accumulate calcium during activation by visual motion stimulation. The spatiotemporal pattern of calcium distribution shows the following characteristics: (i) calcium accumulation is first spatially restricted to those dendritic branches that are depolarized by the retinotopic input, (ii) during ongoing motion stimulation calcium may also accumulate throughout the cell and, in particular, in regions that do not receive direct synaptic input. These experiments successfully monitor the intracellular distribution of activity-dependent ions in visual interneurons of living animals stimulated by their natural synaptic input.