Despite the fact that human immunodeficiency virus (HIV) is transmitted primarily by sexual contact, the biology of the sexual transmission of HIV is poorly understood. Simian immunodeficiency virus (SIV) can be transmitted to female rhesus macaques by placing cell-free virus into the vaginal canal, and SIV can be isolated from the vaginal secretions of infected rhesus macaques. The authors examined the genital tracts from 16 chronically infected female rhesus macaques and localized SIV-infected cells using in situ hybridization and immunohistochemistry. SIV-infected cells were found in the genital tract of 13 of the 16 animals examined, and in most cases the SIV-infected cells were located in the submucosa of the cervix and vagina. However, SIV-infected cells were also found in the vaginal epithelium. SIV-infected cells were more common in sites of inflammation than in normal areas. These findings suggest that SIV gains access to genital tract secretions from the cervix and vaginal epithelium.