Whole-cell recordings in brainstem slices revealed postnatal changes in passive and firing properties in the rat caudal nucleus tractus solitarii (cNTS) neurons. Membrane potential, threshold for Na+ spike and degree of sag were unchanged during development. In the adult, the rheobase was twice that found at birth. The input resistance decreased over the period studied, while time constants declined markedly after the third postnatal week. At all postnatal ages, Na(+)-dependent action potentials (APs) were elicited in response to depolarization. Nevertheless, AP duration gradually decreased by 40% over the developmental period studied. Spike amplitude was smaller at birth than at any other ages and reached a peak two weeks after birth. At all ages, Na(+)-dependent APs were blocked by application of tetrodotoxin. Full APs were replaced by an initial slow oscillation in young cells and by oscillations in older cells. The TTX-resistant oscillations were altered by cobalt (2 mM) and cadmium (100 microM). The spike afterhyperpolarization (AHP) was not altered during development, but was observed in less neurons in adult cells when measured at a holding potential of -60 mV. Neurons were subdivided into one of three classes based on their responses to a hyperpolarizing prepulse: 1) post-inhibitory rebound (PIR) cells, 2) delayed excitation (DE) cells and 3) NON cells expressing neither PIR nor DE. The relative proportions of different cell types varied with age. The mean maximum duration of DE increased three times. Voltage-clamp experiments revealed that the DE was due to the activation of an A-current. In addition, a three-fold increase in its inactivation rate was observed postnatally. The physiological significance of these results is discussed.