- The fine structure of a physiologically identified motor neuron in the segmental ganglion of the leech central nervous system and the morphology of synapses on it were studied after injection of the fluorescent dye Procion yellow as a marker. The injected cell and its processes within the neuropil were located in thick or thin sections with fluorescence optics after initial fixation with glutaraldehyde and brief treatment with osmium tetroxide. The same or adjacent thin sections could then be examined in the electron microscope. Comparison with uninjected cells showed that the general features of the injected cell are retained although some organelles are distorted. The main features of the geometry of this neuron are the same from animal to animal: a single large process runs from the soma through the neuropil to bifurcate and enter the contralateral roots. Within the neuropil the main process gives off long branches (up to 150 micro), but these are greatly outnumbered by short branches and spines, one or a few microns in length, which were not appreciated in previous light microscope studies after injection of Procion yellow. Serial thin sections of selected areas along the main process within the neuropil showed that there are synapses on most of the shorter branches and spines; occasional synaptic contacts were also made on the main process itself and on longer branches. At least two morphologically distinct types of synapse could be recognized. A minimum estimate of the total number of synapses on the motor cell is 300, based on their occurrence in reconstructed segments.