A mild non-progressive cognitive defect is a feature of the fatal X-linked disease, Duchenne muscular dystrophy. Recent studies have identified the genetic defect and the resulting loss of the protein dystrophin, and shown that dystrophin messenger RNA and protein are present in normal brain tissue. We have performed western immunoblotting and fluorescence immunocytochemistry using a sensitive antibody made against a large fragment of the dystrophin molecule to study the regional, cellular and subcellular distribution of dystrophin in the mammalian brain. The brains of B10 (control) and mdx (dystrophin deficient null mutant) mouse brain were compared on a point-by-point basis to verify that only dystrophin and not autosomal dystrophin related protein or cross-reacting proteins were being identified. In addition three murine neurologic mutants, nervous, lurcher, and weaver, were studied to refine the localization of dystrophin. In western immunoblots, dystrophin is present in all regions of the brain and in greatest abundance in the cerebellum. Dystrophin, as demonstrated in immunofluorescence, is present in neurons, but not in glia or myelin, and forms punctate foci associated with the plasma membrane of perikarya and dendrites, but not axons. While dystrophin is abundant in cerebral cortical neurons and cerebellar Purkinje cells, it is absent from most subcortical neurons, the granule cells of fascia dentata, and cerebellar neurons other than Purkinje cells. The absence of dystrophin in the cerebellum of the Purkinje cell deficient mutants nervous and lurcher, and its presence in the granule cell deficient mutant weaver indicate that dystrophin is a component of Purkinje cells rather than closely apposed afferents to those cells. The distribution and localization of dystrophin suggests a role in organizing the plasma membrane, possibly as an anchor of the postsynaptic apparatus, a possible basis for the cognitive defect in Duchenne dystrophy.