Micromolar zinc potentiates neuronal nicotinic acetylcholine receptors (nAChRs) in a subtype dependent manner. Zinc potentiates receptor function even at saturating agonist concentrations, without altering receptor desensitization rate. Potentiation could occur through an increase in the number of available receptors, an increase in single channel current amplitude, or an increase in single channel open probability. To distinguish among these possibilities, we examined rat neuronal nAChRs expressed in Xenopus oocytes. Blockade of a large fraction of ACh activated 44 or 42 receptors by the open channel blocker hexamethonium failed to change the extent of potentiation by zinc, suggesting that zinc does not change the number of available receptors. The single channel amplitudes of ACh (1µM) activated 44 receptors in outside-out patches were similar in the absence and presence of 100µM zinc (3.0±0.1 pA and 2.9±0.1 pA, respectively). To determine the effect of zinc on single channel open probability, we examined 44 receptors in cell-attached patches. The open probability at 100nM ACh (0.011±0.002) was increased 4.5 fold by 100µM zinc (0.050±0.008), accounting for most of the potentiation observed at the whole cell level. The increase in open probability was due to an increase in burst duration, which increased from 207±38 msec in the absence of zinc, to 830±189 msec in the presence of zinc. Our results suggest that potentiation of neuronal nAChRs by zinc is due to a stabilization of the bursting states of the receptor.