The functional significance of gap junctional coupling among neurons is poorly understood. We are studying gap junctions among spinal motor neurons as a model for understanding roles of interneuronal gap junctional communication during development and after injury. Electrical and dye coupling is widespread among neonatal motor neurons but is transient, disappearing by the end of the first postnatal week. Reverse transcription polymerase chain reaction (RT-PCR) analysis, in situ hybridization and immunohistochemistry show that five rodent connexins, Cx36, Cx37, Cx40, Cx43 and Cx45, are expressed by developing motor neurons. These gap junction proteins remain expressed in some motor neurons through adult life, with the exception of Cx40, whose expression appears to decrease shortly after birth. After nerve injury in adult animals, motor neurons once again become dye coupled, and this appears to occur without dramatic changes in connexin expression. The transient gap junctional coupling present among developing motor neurons, which is re-capitulated after axotomy, may mediate electrical or biochemical signaling that shapes neuronal function.