the circumferentially projecting axons of DII neurons achieved full length by P10-11, but the orally and anally projecting axons of uniaxonal neurons grew significantly between P1011 and adult. Neither increase closely matched the increase in size of the small intestine. Uniaxonal neurons underwent major changes in dendritic morphology. While most adult uniaxonal neurons (>70%) had only lamellar dendrites, P0 uniaxonal neurons rarely had lamellar dendrites but have filamentous dendrites instead. Electrophysiological properties of postnatal DII neurons differed markedly from their adult counterparts. At P0, DII neurons rarely had AHPs; and at P10-11, AHPs were only evoked by trains of action potentials. Postnatal DII neurons had prominent Ca2+-mediated afterdepolarizing potentials (ADPs, up to 70 mV in amplitude from a starting membrane potential of -100 mV) and rarely exhibited slow EPSPs. Adult AH/DII neurons displayed AHPs and slow EPSPs, and only a small ADP. Like adult S neurons, at P10-11, S neurons exhibited fast EPSPs which were mainly mediated by nicotinic receptors. Conclusions: This is the first study to examine the postnatal development of electrophysiological andmorphological properties in the intact ENS. At P0, functional synapses are present and neurons can be differentiated electrophysiologically and morphologically into two classes resembling adult classifications, but major modifications occur later in development. When compared their adult counterparts, P10-11 DII neurons are mature morphologically, but not electrophysiologically, while the morphology of S/uniaxonal neurons remains immature.