Ion permeation properties of the mouse e2/zeta1 NMDA receptor channel expressed in Xenopus oocytes were studied using the outside-out patch-clamp technique. In symmetrical Na+ solutions, the single-channel I-V relations were almost linear at low electrolyte concentrations, but rectified inwardly for Na+ concentrations above 50 mm. In symmetrical Na+ solutions, the "zero-current conductance" increased with Na+ concentration and saturated according to a hyperbolic curve, the half-maximal saturating activity, KM(Na), being 14.2 mm and the maximal conductance, Gmax(Na), 53.9 pS. When Ca2+ was present with Na+ in the external solution, the single-channel current was lower than in pure Na+, although the reversal potential indicated a higher permeability for Ca2+ than for Na+. Using ion activities, PCa/PNa was found to be about 17. The I-V data were fitted with a model based on the Eyring's rate theory, assuming a one-ion pore with three energy barriers and two sites. The KM(Ca) and Gmax (Ca) were 76.5 microm and 21.2 pS, respectively. According to the estimated rate constants, KM for Ca2+ is mainly determined by the binding strength of a site located 80% away from the channel opening at the external membrane-solution interface, a position similar to that postulated previously for the Mg2+ blocking site.