We have studied radioautographically the distribution and fate of 3H-glycoproteins within the single identified neurons L10 and R12 of Aplysia californica after intrasomatic pressure injection of [3H]fucose. Silver grains were localized to intracytoplasmic membranes in both cell body and axon 3 h after injection of the cholinergic neuron L10. Grains also appear at this time over presumptive synapses. In the cell body the Golgi apparatus was labeled, as were vesicles, multivesicular bodies, pigment granules, smooth endoplasmic reticulum, mitochondria and peroxisomes. The Golgi apparatus is the most intensely labelled organelle (relative specific activity 9,5). Over 50% of the silver grains are associated with the Golgi apparatus and with vesicles. In the axons, vesicles were labeled most intensely, having a relative specific activity of 40.4 (% silver grains/% area), an intensity 10 times that of similar appearing somatic vesicles, and 4.5--10 times that of other organelles (multivesicular bodies, mitochondria, smooth endoplasmic reticulum) in the axon. At least 32% of the silver grains are associated with vesicles. It appears that the biosynthetic machinery of these neurons is heavily involved in the production of vesicle membrane destined for transport along axons and to terminals. The preponderant labeling of vesicles in the axon parallels the rapid and perferential transport of glycoprotein components described by Ambron et al. and may indicate that specific glycoprotein molecules can be identified as components of these vesicles. After injection of the cholinergic neuron R2 transport of radioactivity was restricted to the axonal tree of the injected neuron. After injection of L10, one other neuron was invariably labeled. By varying the conditions of injection, as many as 5 other neuron cell bodies could be labeled. These are located in the position of cells known to be electrically coupled to L10, and they probably became labeled by transneuronal movement of fucose across electrotonic junctions. Since restriction of label to the injected neuron is easily determined in each experiment, this technique makes possible the identification of chemical and perhaps electrical synapses of identified cells with optimal preservation of fine structure.