We have used single-channel recording techniques to investigate the properties of sodium-activated potassium channels (KNa channels) in cultured rat olfactory bulb neurons, and in large neurons in the mitral cell layer of thin slices of olfactory bulb. Ion channels highly selective for potassium over sodium and chloride, and requiring 10–180 mM internal sodium (Nai) for their activation, were present in approximately 75% of inside-out membrane patches detached from cultured olfactory bulb neurons. Most of these patches contained several KNa channels. KNa channels were seen in cell-attached patches only when Nai was raised by including veratridine in the extracellular medium. Preincubation of the cell in TTX or removal of extracellular sodium prevented this effect of veratridine, confirming that the channels observed under these conditions were indeed KNa channels. Lithium did not substitute for Nai in activating these channels. With 150 mM potassium on both sides of the membrane, KNa channels had a single- channel conductance of 172 pS, and at least two subconducting states were observed in addition to this fully open state. Under these ionic conditions, the channels exhibited linear fully open channel current- voltage curves over the potential range of -100 to 0 mV. At voltages more positive than the potassium equilibrium potential, the single- channel currents exhibited inward rectification as a result of sodium block of outward potassium current. The channels opened in bursts, during which they fluctuated between the fully open and closed states, and the substates. Between bursts they sometimes entered a long-lived inactive state that could last for up to several minutes. In addition, KNa channels in the detached patches exhibited rundown, a progressive irreversible loss in activity, over a time course that varied from less than 1 min to longer than 1 hr. Rundown of KNa channel activity in cell- attached patches (in the presence of veratridine) did not occur, suggesting that some intracellular factor necessary for KNa channel activity is lost when the membrane patch is detached from the cell.