Intact outer segments isolated from bovine retinas (bovine ROS) display a high activity of Na+-Ca2+ exchange, and Na+-Ca2+ exchange appears to be the only functional ion transporter present. Here we demonstrate for the first time that Na+-Ca2+ exchange requires and transports K+ from the following observations. 1) Na+-Ca2+ exchange in bovine ROS required the simultaneous presence of K+ and Ca2+ on one side of the membrane and the presence of Na+ on the other side. 2) Na+-stimulated Ca2+ release from bovine ROS was accompanied by an equally large release of K+. We used the electrogenic protonophore carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) as an added electrical shunt; in the intact rod cell, electrogenic Na+-Ca2+ exchange is shunted by K+ channels present in the rod inner segment. In the presence of FCCP, an inward Na+-Ca2+ exchange current was accompanied by an outward current of protons with a stoichiometry of 1 H+/Ca2+; in the absence of FCCP, no Na+-induced proton current was observed. Addition of FCCP did not uncouple Na+-induced K+ release from Na+-induced Ca2+ release. We conclude that Na+-Ca2+ exchange in bovine rod photoreceptors operates at an electrogenic stoichiometry of 4 Na+:(1 Ca2+ + 1 K+). In isolated ROS and in the absence of an external electrical shunt, Na+-Ca2+ exchange operated at an electroneutral stoichiometry of 3 Na+:(1 Ca2+ + 1 K+).