NF1 patients display CNS abnormalities including learning disabilities, clumsiness, astrocytomas, and abnormalities on magnetic resonance imaging exams. To determine whether the cellular and neuroanatomical distribution of neurofibromin reveals possible function for neurofibromin in the brain, we stained rat brain tissue sections with anti-neurofibromin antibodies. Neurofibromin is highly enriched in large projection neurons, such as cortical and hippocampal pyramidal cells and cerebellar Purkinje cells. Neurofibromin is present in cell bodies and in axons, but is highly enriched in dendrites. Immunoelectron microscopic analysis demonstrates that NF1 is associated with smooth vesiculotubular elements and cisternal stacks and with multivesicular bodies in the cell body and dendrites, but not with the plasma membrane, nucleus, nuclear envelope, Golgi apparatus, mitochondria, or rough endoplasmic reticulum. The preferential localization of neurofibromin to the smooth endoplasmic reticulum, together with evidence that neurofibromin modulates ras GTPase activity, suggests that some, if not all, of the CNS manifestations of NF1 might result from the altered expression of neurofibromin in neurons, perhaps through disruption of Ca2+ signaling, translocation of organelles, or endocytic pathways.