Following a single injection of horseradish peroxidase (HRP) into the superior cervical ganglion (SCG) of the rabbit, the uptake and anterograde transport of this label was confirmed in the ganglion cell bodies, postganglionic axons, and preterminal and terminal ending axons in the ciliary processes of the eye. From the same injection site the intraaxonal HRP reaction product was demonstrated in myelinated axons, presumably by retrograde transport. Intracytoplasmic HRP was identified in large, single membrane-bound, dense vesicles predominantly in perinuclear orientation. Intraaxonal HRP appeared throughout, either within single membrane-bound round or oblong vesicles of variable sizes and densities. Frequently, the HRP vesicles in the axons revealed elaborate membranous subunits. A limited number of whole axons or axon fascicles were diffusely stained with HRP reaction product at or near the injection site. This phenomenon may be the result of membrane injury to neurons. The HRP label was found in small amounts in axons and terminals in the ciliary processes of the eye as early as 4 h following injection into the SCG, indicating a rapid anterograde transport of HRP from a single extracellular source. Likewise the HRP label disappeared from the ganglion cell bodies and processes by the 6th day following injection. Presence of numerous HRP-labeled myelinated and non-myelinated axons in the SCG confirms the bidirectional transport of HRP in the sympathetic nervous system.