We investigated the cellular and subcellular distributions of neuregulin tyrosine kinase receptor ErbB4 in the postnatal rat frontal cortex and hippocampus by light-, confocal- and electron-microscopic immunocytochemistry. At birth, ErbB4-immunoreactivity (ErbB4-IR) was prominent in the apical cytoplasm and dendrites of cortical plate neurons and hippocampal pyramidal cells. Throughout postnatal development and in adulthood, ErbB4-IR in both regions remained confined to the somatodendritic compartment of neurons, which increased in number to reach the adult pattern by the end of the first postnatal month (P30). At all ages examined, double-labeling experiments revealed that ErbB4-IR always co-localized with the neuronal marker neuronal nuclei (NeuN) and never with glial markers Nestin or glial fibrillary acidic protein (GFAP). Immunoperoxidase labeling at the ultrastructural level confirmed the exclusive localization of ErbB4-IR in somatodendrites, and notably in dendritic spines. Immunogold labeling showed preponderant ErbB4-IR in the cytoplasm, where it was associated with microtubules. Furthermore, ErbB4-IR was abundant in the nucleus of adult cortical and hippocampal neurons, suggesting a role for ErbB4 nuclear signaling in the brain beyond embryonic development. Taken together, these results show that ErbB4 is expressed by neuronal somatodendrites in cerebral cortex and hippocampus from birth to adulthood, and support a role for neuregulins in dendritic growth and plasticity.