Calmodulin (CAM), as well as other Ca2+ binding motifs (i.e., EF hands), have been demonstrated to be Ca2+ sensors for several ion channel types, usually resulting in an inactivation in a negative feedback manner. This provides a novel target for the regulation of such channels. We have designed peptides that interact with EF hands of CaM in a specific and productive manner. Here we have examined whether these peptides block certain Ca2+-permeant channels and inhibit biological activity that is dependent on the influx of Ca2+. We found that these peptides are able to enter the cell and directly, as well as indirectly (through CAM), block the activity of glutamate receptor channels in cultured neocortical neurons and a nonselective cation channel in Jurkat T cells that is activated by HIV-1 gp120. As a consequence, apoptosis mediated by an influx of Ca2+ through these channels was also dose-dependently inhibited by these novel peptides. Thus, this new type of Ca2+ channel blocker may have utility in controlling apoptosis due to HIV infection or neuronal loss due to ischemia.