Previous immunofluorescence analyses in mice and rats showed a mainly astrocytic expression of the gap junction protein connexin43 (Cx43) in brain. However, in situ hybridization of murine brain sections suggested strong expression of Cx43 mRNA in hippocampal and cortical pyramidal neurons and Purkinje cells. These findings contrast with recent immunoelectron microscopic studies that excluded prominent Cx43 protein expression in neurons. Both contrasting results could be explained by post-transcriptional control mechanisms. Here we demonstrate by conditional replacement of the Cx43 coding region by a lacZ reporter gene, mimicking transcriptional activity of the Cx43 gene, that Cx43 is not expressed in principal cells of murine brain. This histochemical approach used is not prone to cross-reactivity of mRNA probes or antibodies. Furthermore, we show that in situ hybridization signals, suggested to be specific for Cx43 in mouse neurons, are retained even when the Cx43 coding DNA in neurons is removed by cre-mediated deletion. Our results confirm the previous findings of a mainly astrocytic expression of Cx43 in adult mouse brain and underscore the importance of connexin-deficient mice as controls for in situ hybridization studies. We found no evidence for post-transcriptional control of the Cx43 gene in principal neurons. Thus, the synchronized activity of neuronal networks cannot depend on Cx43 containing gap junctions in these cells.