P2X receptors are cationic-selective ion channels gated by extracellular ATP. There are seven subunits (P2X_1-7), the first six of which are expressed throughout the peripheral and central nervous systems. P2X₇ receptors are rapidly upregulated and activated as a result of inflammatory stimuli in immune cells, where they act not only as cationic channels but uniquely couple with rapid release of proinflammatory cytokines, cytoskeletal rearrangements, and apoptosis or necrotic cell death. The P2X₇ receptor has been termed the cytolytic non-neuronal P2X receptor because it had not been detected in neurons until recently when it has been immunolocalized to several brain regions, particularly the hippocampus, and has been suggested to be involved in presynaptic modulation of transmitter release. Because its expression in brain neurons may have substantial functional implications, we have performed detailed immunocytochemical, immunoblot, and immunoprecipitation studies on brain and non-neuronal tissue using all currently available antibodies. We first examined rats, but staining patterns were inconsistent among antibodies; we therefore studied mice for which there are two P2X₇ knock-out mice constructs available, one expressing the LacZ transgene. We found that P2X₇ receptor protein is strongly and reliably detected in the submandibular gland and lung of wild-type mice but not in either of the P2X₇^-/- mice. However, we failed to find evidence for P2X₇ receptor protein in hippocampal neurons or their input-output projections. Either the P2X₇ protein in the hippocampus is below the limits of detection by the currently available methods or it is not present.