We have estimated the prevalence of coupling between neurons of the rat neocortex during postnatal development. Single intracellular injections of the fluorescent dye Lucifer Yellow CH resulted in dye coupling among 70% of neurons from 1 to 4 days of age. Dye coupling dropped to 30 to 40% by 10 to 18 days and occurred in 20% of injected adult neurons. The number of neurons per dye-coupled aggregate also decreased. Whereas three to seven coupled neurons were common in cortex of 1 to 4 days, aggregates of more than two neurons were exceptionally rare in adults. The frequency of dye coupling did not vary systematically with cortical depth at any age. When chemical synaptic activity was blocked, most 4-day neurons exhibited short latency antidromically evoked depolarizations which were relatively insensitive to repetitive activation and membrane polarization. These depolarizations may represent electrotonically conducted spikes from coupled neurons. No such potentials were found in adult neurons. The results suggest that neuronal coupling is extensive in immature rat neocortex, but that coupling declines at a time just before the numbers of chemical synapses increase most rapidly.