The ZNT3 protein decorates the presynaptic vesicles of central neurons harboring vesicular zinc, and deletion of this protein removes staining for zinc. However, it has been unclear whether only histochemically reactive zinc is lacking or if, indeed, total elemental zinc is missing from neurons lacking the Slc30a3 gene, which encodes the ZNT3 protein. The limitations of conventional histochemical procedures have contributed to this enigma. However, a novel technique, microprobe synchrotron X-ray fluorescence, reveals that the normal 2- to 3-fold elevation of zinc concentration normally present in the hippocampal mossy fibers is absent in Slc30a3 knockout (ZNT3) mice. Thus, the ZNT3 protein evidently controls not only the "stainability" but also the actual mass of zinc in mossy-fiber synaptic vesicles. This work thus confirms the metal-transporting role of the ZNT3 protein in the brain. ?? The Histochemical Society, Inc.