We investigated the developmental expression and cellular resolution of connexin32 and 43 mRNA in the rat brain using in situ hybridization. Utilizing 35S-labelled probes, in situ hybridization was performed on sections of embryonic day 20 and postnatal days 3, 10, 15, 30 and adult brain. Connexin32 mRNA was first detected in brainstem nuclei at postnatal day 3 and in the midbrain at postnatal day 15. The level of this message continued to increase to postnatal day 30 where the level of message reached a plateau or slightly decreased by adulthood. The distribution of signal included the medial vestibular nucleus, dorsal paragigantocellular nucleus and fibre tracts of the midbrain and brainstem such as the cerebellar white matter, spinal trigeminal tract, and decussation of the superior cerebellar peduncle, areas containing predominantly neurons or oligodendrocytes. Connexin43 mRNA was first detected much earlier than connexin32 and was found in the leptomeniges of the E20 brain. It was found at all ages examined and distributed homogenously in the regions of the brain examined, suggesting its presence in astrocytes. The connexin43 riboprobe also hybridized strongly to the ependymal cells of the fourth ventricle and cerebral aqueduct. These results describe the cellular resolution of connexin mRNAs during development and that a differential pattern of expression exists in the various cell populations in the central nervous system.