HIV encephalitis (HIVE) is a neurodegenerative disease seen in approximately one in four terminally infected patients. Macaques infected with the simian immunodeficiency virus develop encephalitis (SIVE) very similar to the human disease. Neurodegeneration in both these conditions occurs from the effects of toxic viral proteins and neurotoxins derived from activated brain macrophages. Activated macrophages in the brain of macaques with SIVE can be labeled in vivo using positron emission tomography (PET) using PK11195, a ligand that binds the peripheral benzodiazepine receptor (PBR). However, the functional significance and mechanisms mediating increased PK11195 binding in activated brain macrophages are not known. Using post mortem tissues from macaques with SIVE and macrophages cell cultures activated with lipopolysaccharide (LPS), we show that [(3)H](R)-PK11195 binding is increased in activated macrophages. Increased [(3)H](R)-PK11195 binding in LPS-activated macrophages was reversed by pharmacologically inhibiting class III phosphatidylinositol-3 kinase (PI3-kinase), but was not altered by inhibiting the mitogen-activated protein kinase (MAP-kinase) pathway. Our results suggest that activated macrophages in lentiviral encephalitis show increased [(3)H](R)-PK11195 binding in a PI3-kinase-dependent fashion which may help elucidate the function of PBR in activated brain macrophages in HIVE and other neuroinflammatory diseases.