ObjectiveTo observe ultrastructural changes of central nerve system and lymph nodes at different stages of SIV infection in rhesus monkeys, explore the pathogenesis of simian AIDS. Methods15 rhesus monkeys, 1 was used as normal control, the other 14 monkeys were infected with SIVmac239 intravenously. Inguinal lymph nodes and hypothalamic tissues were inspected by transmission electron microscopy at 1 week, 2 weeks, 1 month, 2 months, 12 months and 18 months after infection. Results1 week after infection, lymphocytes in lymph node showed pathological changes, manifested as mitochondria swelling with cristae dissolution and autophagies. At time points of 2 weeks and 1 month after infection, lymphocytes showed mitochondria swelling with cristae dissolution. 2 months after infection, the nuclei of the lymphocytes were obviously deformed. At 12 months after infection, a high proportion of lymphocytes had obvious lesions, manifested as cell swelling, most organelles lost their normal appearance; some cells showed features of solubilized necrosis. At 18 months after infection, the lymphocytes were sparse in some areas of the lymph node, the nuclei of the lymphocytes were swelling or had irregular shapes. Central nervous system lesions appeared at 1 week after infection, manifested as nerve fiber swelling. At 2 weeks after infection, the neuron showed mitochondria swelling, rough endoplasmic reticulum degranulation, Nissl bodies disappearing; there were vacuoles in the nerve fibers. At 12 months and 18 months after infection, the lesions of neurons and nerve fibers were increased. ConclusionsIn the early stage of SIV infection, the lesions can be found in lymph node and central nervous system. After maintaining a relatively stable stage, the disease deteriorated, the lesions were increased. The lesions in the central nervous system and lymph node showed some similarity in the development.