The properties of the vestibular Na+–Ca2+ exchanger in mammalian type I vestibular sensory cells were studied using fura-2 fluorescence and immunocytochemical techniques. In the absence of external Na+, the activation of Na+–Ca2+ exchange in reverse mode required the presence of external K+ (K+o) and depended on K+o concentration. Alkali cations Rb+ and NH4+ but not Li+ or Cs+ substituted for K+o to activate the exchange. For pressure applications of 10 mm K+, the contribution of voltage-sensitive calcium channels to the increase in [Ca2+]i was < 15%. The dependence of the exchange on [K+]o was also recorded when the membrane potential was clamped using carbonyl cyanide p-trifluoromethoxy-phenylhydrazone (FCCP) and monensin ionophores. In these conditions, where there was no intracellular Na+, the increase in [Ca2+]i was completely blocked. These physiological results suggest that in reverse mode, Ca2+ entry is driven by both an outward transport of Na+ and an inward transport of K+. The dependence of the vestibular Na+–Ca2+ exchanger on K+ is more reminiscent of the properties of the retinal type Na+–Ca2+ exchanger than those of the more widely distributed cardiac type exchanger. Moreover, the immunocytochemical localization of both types of exchange proteins in the vestibular sensory epithelium confirmed the presence in the vestibular sensory cells of a Na+–Ca2+ exchanger which is recognized by an antibody raised against retinal type and not by an antibody raised against the cardiac type.