To investigate the mechanism of simian immunodeficiency virus (SIV) entry into the central nervous system (CNS) and the initial events leading to neuropathogenesis, SIV replication was studied by in situ hybridization in the CNS of 5 Rhesus macaques at 7 days, 1, 2, and 3 months after SIV intravenous inoculation. CNS infection was found to be a frequent and early event, as SIV was detected in the CNS of all the animals studied and as early as 7 days postinoculation. At the earliest stage, the infection localized mainly to perivascular cells. Using combined immunohistochemistry and in situ hybridization, infected cells were shown to express the CD68 marker, suggesting that infected mononuclear phagocytes crossing the blood-brain barrier represent the main source of virus in the CNS. Early viral replication coincided with neuropathologic changes, consisting in gliosis, perivascular infiltrates and rare glial nodules. Immunophenotyping of brain tissue showed that increased macrophage infiltration, microglial reactivity and MHC class II induction occurred within the first week of infection, indicating a possible immunopathologic mechanism in early CNS pathogenesis.