The sodium channel beta 1 subunit (Na beta 1) is a component of the rat brain voltage-dependent sodium channel. We have used nonradioactive in situ hybridization cytochemical techniques to demonstrate that transcript levels of Na beta 1 are differentially upregulated during postnatal development of several CNS regions, with selective labeling of specific neuronal populations. In the hippocampus, labeling of the pyramidal cell layer (particularly in the CA3 region) and dentate granule cells was initially observed at postnatal day 2 (P2) and P10, respectively, and became progressively more intense with maturation. Labeled cells were first observed in the hilus at P10. In the developing cerebellum, transient labeling was observed in the external granule cell layer beginning at P1 while label increased in the internal granule cell layer up to P21. Purkinje cells showed significant label beginning at P4 and increasing up to P21. Weak signal was seen in neurons of deep nuclei at P1 and increased up to P21. Na beta 1 labeling in the spinal cord was first observed in the ventral horn at P2, and the intensity of labeling in these large motoneurons gradually increased. In addition, there was a ventral-dorsal gradient in this region, with label appearing subsequently in neurons of Rexed laminae IX, VII and VIII, and in the dorsal horn (Rexed laminae I-VI). In these regions, the labeling reached a plateau within the first 2-3 weeks after birth and persisted into the adult rat. The time course and regional heterogeneity of Na beta 1 expression are consistent with the hypothesis that the expression of mature Na+ channels, including Na beta 1, contributes to the development of circuitry that supports complex patterns of electrogenesis.