The present study describes ultrastructural changes in the ciliary ganglia of the cat and monkey following preganglionic axotomy. At 3, 5 and 7 days after operation, the nucleus of some neurons was irregular, with prominent indentations, and displaced to the periphery of the neuron. The surface of most neurons was irregular. Neurofilaments and glycogen-like granules were much increased in some neurons. At 21 and 28 days after operation, neurons again appeared normal. Dendritic profiles, packed with many mitochondria and glycogen-like granules, could often be observed from 3 days after operation. In longitudinal section such profiles represented expanded trunks of dendrites; dilated mitochondria and dense bodies were sometimes encountered within them. At later stages after operation, some of these profiles were synaptically contacted by, or closely associated with, axon terminals. In myelinated axons, mitochondria and glycogen-like granules were also increased in number and dilated profiles and dense bodies were found within the axoplasm. In unmyelinated axons, dilated profiles and myelin-like figures were present, as were vesiculo-tubular structures and dense bodies. Electron-dense and -lucent changes could both be observed in myelinated and unmyelinated axons. Almost all the axon terminals were affected 3 days after operation. Within such degenerating axon terminals, the synaptic vesicles had accumulated to form one or several clumps; sometimes the degenerating axon terminals had undergone filamentous hyperplasia. At 45 days after operation, hardly any axon terminals were encountered. Non-neuronal cells, including satellite cells, macrophages and Schwann cells, were actively involved in removing degenerating axons and other cell debris.