Gap junction coupling between neurons is important for the temporal and spatial co-ordination of neocortical development and can be visualised by dye-coupling. Neuronal dye-coupling in the rat neocortex is extensive during the first 2 postnatal weeks and diminishes rapidly thereafter. We used RT (reverse transcriptase)-PCR to investigate the time-related changes in mRNA expression for the connexins (Cx) Cx 26, Cx 30, Cx 32, Cx 36, Cx 37, Cx 40, Cx 43, Cx 45 and Cx 46 as well as for beta-actin and GAPDH in rat neocortex during the first 6 postnatal weeks. The time courses for mRNA expression for GAPDH, Cx 30, Cx 36 and Cx 43 were also investigated by northern blotting. Cx 30 and Cx 45 mRNA abundance showed no time-dependent changes during the early postnatal period. The relative abundance of Cx 32, Cx 43 and Cx 46 mRNA increased significantly during the first 2-3 weeks and then remained relatively constant during weeks 3-6. The relative abundance of Cx 26, Cx 36, Cx 37 and Cx 40 mRNA also increased significantly during the first 10-15 postnatal days but then declined significantly from their peak values during weeks 3-6. beta-actin mRNA expression showed no time-related changes but GAPDH mRNA expression increased significantly during the first postnatal week, then remained constant. The time-dependent changes in mRNA relative abundance for GAPDH, Cx 36 and Cx 43 determined by northern blotting corroborate the results from the RT-PCR study. None of the Cx exhibited time-dependent changes in mRNA expression in homogenates of rat neocortex which parallel the changes in neuronal dye-coupling during postnatal development.