The distribution of presynaptic endings on the surfaces of autonomic ganglion cells was mapped in living mice after intravenous administration of a styryl pyridinium dye. The staining and imaging techniques did not appear to damage the ganglion cells, or the synapses on them; these procedures could therefore be repeated after an arbitrary period. Observations of the same neurons at intervals of up to 3 weeks indicate that the pattern of preganglionic terminals on many of these nerve cells gradually changes.