We have investigated the neurotropism of feline immunodeficiency virus (FIV) in naturally and experimentally infected cats. Antibodies to FIV were detected in the cerebrospinal fluid (CSF) of 9 of 10 naturally infected cats, and the virus was cultured from the CSF of 5 of 9 of these cats. After experimental intrathecal or intra-bone-marrow inoculation, FIV antibodies were detected in CSF, as were CSF pleocytosis, increased IgG concentration, and elevated CSF IgG index. Brain lesions consisting of perivascular mononuclear cell infiltrates and both glial nodules and diffuse gliosis were observed in the midbrain and thalamus 7 months after inoculation. Virus was recovered by primary culture of brain tissue from several brain regions (cerebral cortex, caudate nucleus, midbrain, cerebellum, rostral and caudal brainstem) but was not recovered from choroid plexus. In vitro, FIV infected primary cultures of feline astrocytes and brain macrophages. Infection of astrocytes resulted in early syncytium formation, production of infectious virions, and eventual cell death. In brain macrophages, FIV infection was noncytopathic. Productive infection of feline neurons or oligodendrocytes was not observed. We conclude that FIV is a neurotropic lentivirus and that FIV infection of feline CNS may serve as a useful model for study of human immune deficiency virus infection of the human CNS.