The synchronisation of the discharges recorded from left and right phrenic nerves in the adult rat is produced in part by shared excitation from a common premotor neurone population. However, such synchronisation has not been examined for hypoglossal motoneurones in adult rats, or for phrenic and hypoglossal motoneurons in neonatal in vitro preparations. In adult rats, cross-correlograms computed between the inspiratory discharges of the left and right phrenic nerves, and the left and right hypoglossal nerves displayed central peaks with half-amplitude widths of 1.4+/-0.1 and 1.7+/-0.1 ms (mean+/-SE), respectively. We interpret these as evidence for common excitation. However, such central peaks were absent in the same cross-correlograms computed for neonatal in vitro preparations, although central peaks were observed in cross-correlograms computed between the discharges recorded from adjacent phrenic nerve rootlets. We conclude that, in the adult rat, left and right hypoglossal nerve discharges are synchronised by excitation from a common premotor neurone population, as for the phrenic nerves, but this type of synchronisation is undetectable in neonatal in vitro preparations. We speculate that the differences between the adult and neonatal preparations are due to developmental changes in respiratory drive transmission pathways.