Gap junction-mediated dye-coupling between developing neocortical pyramidal cells occurs in high levels until postnatal day P9 and gradually decreases to near zero levels in the following 7 days. Electrophysiological data demonstrating electrical coupling between pyramidal neurons during this period are still lacking. We therefore performed patch clamp recordings from pairs of layer II/III pyramidal cells in rat neocortical slices. Pyramidal neurons were visualized using an IR-DIC microscope and classified by their morphology and their regular spiking characteristics. In P6-8 rats, 14 of 16 pairs of neurons demonstrated electrical coupling. Injections of de-and hyperpolarizing pulses into one cell elicited corresponding voltage deviations in the other cell. An apparent coupling coefficient was determined and found to range between 0.03 and 0.08. The distances between the somata of the recorded cells varied between 15 and 30 mm suggesting that electrical coupling occurs within the dendrites. In P13-17 rats, electrical coupling between pyramidal neurons was absent (n=9 pairs). Single cell RT-PCR analysis revealed the expression of Connexin-36 and Pannexin-2 mRNA in pyramidal neurons (n=40). Thus, developing pyramidal cells display electrical coupling which correlates with the previously reported development dependent dye- coupling. Supported by DFG Su 104/3-3.