Inhibitor-2 (I-2) is the cytosolic regulatory subunit of type 1 protein phosphatase. We cloned and sequenced a cDNA for rat I-2 from rat brain cDNA library. By in situ hybridization histochemistry, I-2 mRNA was expressed throughout the adult rat brain (on the postnatal 7th week) at various hybridization intensities: high expression signals were detected in the hippocampal formation, olfactory neuronal layers, choroid plexus and cerebellar Purkinje and granule cell layers. Moderate expression signals were detected homogeneously through the layers II-VI of the neocortex. On the embryonic day 15, 1-2 mRNA was expressed moderately in both ventricular and mantle zones of the fore-, mid-, hindbrain and spinal cord, suggesting the possible involvement of I-2 together with type 1 protein phosphatase (PP-1) in the regulation of various early neuronal events including mitosis, migration and differentiation.