Quantum dots (QDs) have shown great promise for applications in biology and medicine, which is being challenged by their potential nanotoxicity. Reactive oxygen species (ROS) produced by QDs are believed to be partially responsible for QD cytotoxicity. Cytoplasmic Ca(2+) plays an important role in the development of ROS injury. Here we found unmodified cadmium selenium (CdSe) QDs could elevate cytoplasmic calcium levels ([Ca(2+)](i)) in primary cultures of hippocampal neurons, involved both extracellular Ca(2+) influx and internal Ca(2+) release. More specifically, verapamil and mibefradil (L-type and T-type calcium channels antagonists, respectively) failed to prevent extracellular Ca(2+) influx under QD insult, while omega-conotoxin (N-type antagonist) could partially block this Ca(2+) influx. Surprisingly, this Ca(2+) influx could be well blocked by voltage-gated sodium channels (VGSCs) antagonist, tetrodotoxin (TTX). QD-induced internal Ca(2+) release could be avoided by clonazepam, a specific inhibitor of mitochondrial sodium-calcium exchangers (MNCX), and also by TTX. Furthermore, dantrolene, an antagonist of ryanodine (Ry) receptors in endoplasmic reticulum (ER), almost abolished internal Ca(2+) release, while 2-APB [inositol triphosphate (IP(3)) receptors antagonist] failed to block this Ca(2+) release, indicating that released Ca(2+) from mitochondria, which was induced by extracellular Na(+) influx, further triggered much more Ca(2+) release from ER. Our results imply that more research on the biocompatibility and biosafety of QD is both warranted and necessary.