Abstract Purified secretory vesicles isolated from bovine neurohypophyses take up Na + under the same circumstances where an efflux of Ca 2+ takes place, suggesting a Na + Ca 2+ exchange. Potassium cannot substitute for Na + in this process. Also, a Ca 2+ Ca 2+ exchange can occur. Inhibiting the latter process by Mg 2+ allowed to estimate an apparent K M of 0.7 μM free Ca 2+ and a maximal uptake of 1.5 nmol × mg protein −1 × min −1 Ca 2+ in exchange for Na +. The vesicles did not contain plasma membrane marker ( Na + K + ATPase) as shown by distribution analyses on the density gradients on which they were purified. Similarly, distribution studies also showed that no other ATPase activity could be detected in the purified vesicle fraction. It is concluded that a Na + Ca 2+ exchange is operating across the secretory vesicle membrane and that it is not directly dependent on ATP hydrolysis.