The ultrastructural changes in the ciliary ganglion of the cat and monkey after postganglionic (short ciliary) nerve section are described. They were similar in both cat and monkey. The nucleus became irregular with prominent indentations on the 3rd day after operation and was peripherally located in the neuron from the 5th postoperative day; by 1 month postoperatively, however it had reverted to its former round or oval shape and became centrally located. In the cytoplasm, glycogen-like granules were increased. At 5 d after operation, profiles of dilated endoplasmic reticulum (ER) and vacuoles were observed at the periphery of some neurons. At 7 d after operation, there was an increase in the number of dense bodies in the cytoplasm of the neurons. By 1-3 months after operation, most of the neurons appeared normal, but there was an increase of profiles of the Golgi apparatus in the perinuclear region. In the cat, there was an increase in profiles of glycogen-like granules associated with ER stacks. In the monkey, the glycogen-like granules were randomly distributed in the cytoplasm. Dendritic profiles with large numbers of mitochondria and glycogen-like granules were observed as early as the 1st postoperative day and were increased on the 3rd postoperative day. Some of the profiles were contacted synaptically by axon terminals. Such profiles measured between 0.8 to 4.5 microns in the cat and 1.1-7.0 microns in the monkey. The number of mitochondria per dendritic profile in cross section varied from 8 to 155 in the cat and 28 to 230 in the monkey. Dilated and almost empty mitochondria and dense bodies were encountered in such profiles. Dendritic profiles packed with mitochondria were still present 1-3 months after operation, but were reduced in number and dense bodies were only occasionally observed. Some of the myelinated and unmyelinated axons underwent degeneration, but no ultrastructural change was observed in the axon terminals. In some axonal profiles, vesiculotubular structures were also present. At 7-14 d after operation, some of the satellite cells became less electron dense. At this stage, some of the cells contained phagocytosed electron-dense profiles resembling degenerating myelinated axons and cellular debris. Macrophages were encountered frequently 3-7 d after operation; some of these were believed to be derived from circulating monocytes since they were occasionally observed to penetrate the vascular wall. Most of the engulfed material in the cytoplasm of the macrophages were myelin-like figures and vesiculated profiles.(ABSTRACT TRUNCATED AT 400 WORDS)