Twelve sheep experimentally infected with Chlamydia psittaci during pregnancy either aborted or gave birth to weak, low-birth-weight lambs as a result of uteroplacental infection. Subsequently, these ewes excreted chlamydiaI antigen from their reproductive tracts during estrus. About 1 year after pregnancy failure, immunohistochemistry and polymerase chain reaction were used to examine sections of vagina, uterus, and oviduct for evidence of C. psittaci. Four noninfected control ewes were similarly examined. C. psittaci antigen or DNA was detected in vagina, uterus, and oviduct samples from chronically infected ewes. Endometrial cells in the basal stroma were the predominant site of infection. There was no obvious evidence of pathology associated with persistent infection, but increased numbers of plasma cells and intraepitheliallymphocytes were detected in the uterus. C. psittaci is a naturally occurring reproductive infection in sheep that persists following primary infection. Therefore, sheep provide an excellent model to study the host-parasite interactions that occur during natural infection and subsequent persistence.