The membrane-impermeable chelator CaEDTA was introduced extracellularly among neurons in vivo and in vitro for the purpose of chelating extracellular Zn²⁺. Unexpectedly, this treatment caused histochemically reactive Zn²⁺ in intracellular compartments to drop rapidly. The same general result was seen with intravesicular Zn²⁺, which fell after CaEDTA infusion into the lateral ventricle of the brain, with perikaryal Zn²⁺ in Purkinje neurons (in vivo) and with cortical neurons (in vitro). These findings suggest either that the volume of zinc ion efflux and reuptake is higher than previously suspected or that EDTA can enter cells and vesicles. Caution is therefore warranted in attempting to manipulate extracellular or intracellular Zn²⁺ selectively.