Intraocular injection of vinblastine sulfate (VLB) at a dose of 10–500 μg disrupts microtubules and blocks rapid axoplasmic transport in the rabbit retinal ganglion cells (GCs). A dose of 500 μg VLB produces characteristics Vinca alkaloid paracrystals in the GCs by 24 h, and unusual paracrystals are noted within optic nerve terminals in the contralateral superior colliculus at survival times of 3, 7 and 14 days. These paracrystals are composed of hexagonally arranged unit structures measuring approximately 55 nm in diameter, and each unit is resolved as a central 25 nm tubule circled by five 15 nm subunits. The paracrystals are surrounded by spherical 50 nm synaptic vesicles, and usually lie adjacent and parallel to the large, pale mitochondria which characterize the optic terminals. The possible significance of the paracrystals is discussed; they may represent an unusual form of neuronal degeneration, an atypical VLB product, or an accumulation of virus particles. Degenerative changes, including enlarged synaptic vesicles and slightly increased background density, were noted in some optic terminals at 7 days following injection of 10–250 μg of VLB. These changes were reversible, and the terminals appeared normal after resumption of rapid axoplasmic transport. These morphologic alterations in the optic terminals may reflect the lack of materials synthesized in the GC perikarya and conveyed to the terminals by rapid axoplasmic transport.