To understand the emergence of excitability in vestibular nuclei neurons, we performed patch-clamp recordings on brain slices to characterize the firing pattern on depolarization and the underlying currents in principal cells of the chick tangential nucleus. This study, on 0- to 3-day-old hatchlings, distinguishes electrophysiologically one main group of principal cells based on their response to depolarizing current pulses (300-400 ms) in current-clamp recordings. This group (90%; n=29) displayed nonaccommodating, repetitive firing on depolarization. The remaining cells fired one action potential at the beginning of the current pulse and then accommodated. In voltage-clamp recordings, a low-threshold, sustained, dendrotoxin-sensitive (DTX; 200 nM) potassium current, I(DS), was identified in both cell groups. In the repetitively firing principal cells, the mean proportion of the DTX-sensitive sustained current contributing to the total outward current was less than 20%. This percentage is significantly less than that reported (45%) in a previous study performed in late chick embryos (E16), in which most of the cells (83%; n=89) were accommodating neurons. Tonic firing is an important electrophysiological feature characterizing most mature, second-order vestibular neurons, since it allows the neurons to process signals from behaviorally relevant inputs. Accordingly, this study contributes toward defining the emergence of the mature pattern of neuronal excitability and the ionic currents involved.