Since neuronal activity is critically related to both intra- and extracellular calcium ion concentrations, there must be a mechanism for regulating the transfer of calcium from the blood into the extracellular environment of the brain. The data presented here support the hypothesis that, within or close to the site of passive permeation of calcium in the choroid plexus, there is a calcium pump capable of recycling the filtered calcium back into the capillary circulation. This pump can be blocked by ruthenium red, a known inhibitor of active calcium transport. The activity of the calcium pump is shown to increase in proportion to the concentration of calcium in the cerebroventricular fluid. Its physiological role therefore would be to insure that the calcium concentration available to the brain will be maintained within a narrow range even in the presence of severe hypercalcemic states.