The expression of connexin43 mRNA was detected in adult rat brains by in situ hybridization methods. Specific digoxigenin riboprobes were generated by in vitro transcription of two PCR-amplified fragments of connexin43 cDNA which lack homology with any other published connexin. Following immunohistochemical detection, the digoxigenin cRNA was found to occur in various neuronal populations including Purkinje cells of the cerebellum, pyramidal cells of the neocortex and the hippocampal formation, as well as granule cells of the dentate gyrus and various neurons of diverse hindbrain nuclei. This ubiquituous expression of connexin43 mRNA in adult neurons, in particular in neocortical pyramidal cells, is surprising insofar as gap junction communication in adult bains has been considered to be confined to specific subpopulations of neurons revealing a high incidence of synchronized electrical activities in contrast to the postnatal brain where interneuronal coupling via gap junctions precedes the formation of chemical transmission. In addition, connexin43 is regarded as being preferentially expressed in astrocytes, although its presence in adult neurons has not definitely been excluded. We propose that adult neurons preserve their capability of expressing functional gap junctions more frequently than presently considered and that connexin43 is a most likely neuronal gap junction protein candidate.