Studies on the identification and characterization of constituents of rat brain synaptic junctions have lead to the isolation of cDNA clones encoding segments of alpha-adducin. These and other studies suggest that adducin, a protein involved in promoting the assembly of actin and spectrin filaments at the plasma membrane, may play a role in dynamic assembly-disassembly processes underlying synaptic plasticity. In order to verify that brain alpha-adducin is indeed a constituent of synaptic structures, we have generated monoclonal antibodies against epitopes in the C-terminal region of alpha-adducin and have determined its spatial and sub-cellular distribution in postnatal day-30 rat brain. Alpha-adducin is found to be highly enriched in regions with high synapse densities of the hippocampus, corpus striatum, cerebral cortex and cerebellum. Immuno-electron microscopic analysis of peroxidase stained sections of the hippocampus and the cerebellum revealed that alpha-adducin is localized at distinct sub-cellular structures. In the CA1 and CA3 regions of the hippocampus alpha-adducin immunoreactivity is found in a distinct subset of dendrites and dendritic spines. In the molecular layer of the cerebellum, a distinct fraction of pre-synaptic terminals of parallel fiber terminals is labeled. In both cases the majority of synaptic structures does not contain adducin. Significant immunoreactivity is also detected in processes of glial cells both in the hippocampus and the cerebellum.