The actions of divalent cations on voltage-gated ion channels suggest that these cations bind to specific sites and directly influence gating kinetics. We have examined some chemical properties of the external divalent cation binding sites on neuronal potassium channels. Patch clamp techniques were used to measure the electrophysiological properties of these channels and Zn ions were used to probe the divalent cation binding site. The channel activation kinetics were greatly (three- to fourfold) slowed by low (2-5 mM) concentrations of Zn; deactivation kinetics were only slightly affected. These effects of Zn were inhibited by low solution pH in a manner consistent with competition between Zn and H ions for a single site. The apparent inhibitory pK for this site was near 7.2. Treatment of the neurons with specific amino acid reagents implicated amino, but no histidyl or sulfhydryl, residues in divalent cation binding.