Evidence from several areas of neuroscience has led to the notion that copper and zinc could be modulators of neuronal excitability. In order to contribute to test this idea, we characterized the changes induced by these divalent metal ions on the extracellularly recorded action potential firing rates of undissociated olfactory epithelium neurons. Our main finding is that at low concentrations, 1-100 nM for Cu(2+) and 1-50 microM for Zn(2+), they induced a concentration dependent increase in the neuronal firing rate. In contrast, at higher concentrations, 1-5 microM for Cu(2+) and 100-500 microM for Zn(2+), they decreased the firing rate. Based on these and previous results of our laboratory we propose that the biphasic effect of Cu(2+) and Zn(2+) exposure on neuronal firing may be explained by the interaction of these ions with high and low affinity sites in sodium channels whose occupancy leads to activation or inhibition of the sodium current, which is consistent with the proposed modulatory role of these metal ions on neuronal excitability.