Intracellular recording techniques were used to characterize changes that take place in rat hypoglossal motoneuronal excitability from early postnatal stages to adulthood. This study focused primarily on the first two weeks of postnatal life, when major changes in the maturation of the neuromuscular system take place. Neonatal hypoglossal motoneurons were identified by their location within the hypoglossal nucleus and by their characteristic electrophysiology. These criteria were supported by antidromic activation and intracellular staining of retrogradely labeled hypoglossal motoneurons. Action potential duration decreased progressively during postnatal development. The reduction was primarily due to a more rapid repolarization, suggesting developmental changes in voltage-dependent potassium conductances. The duration of the calcium-dependent afterhyperpolarization decreased by half during the first two weeks of postnatal life. Changes in subthreshold responses included a decrease in input resistance and an increase in the degree of hyperpolarizing sag and inward rectification with age. Rheobase current was negatively correlated with input resistance, and increased progressively during postnatal development. Membrane time constant decreased almost four-fold over the first two postnatal weeks, suggesting that membrane resistivity is not constant. This decrease in membrane resistivity could account for a large fraction of the change in input resistance and rheobase with age. Thus, the early postnatal development of the rat includes systematic changes in the electrophysiological properties of motoneurons innervating tongue muscles. Some of these modifications are not easily explained by a mere change in neuronal surface area but likely involve changes in the density of expressed ion channels.