A monoclonal antibody was developed to the extracellular domain of the rat P2X4 receptor. The antibody was highly selective among all rat P2X receptor subunits, and recognised only the oligomeric, non-denatured form of the P2X4 receptor. Immunohistochemistry showed an extensive pattern of distribution throughout the central and peripheral nervous systems, the epithelia of ducted glands and airways, smooth muscle of bladder, gastrointestinal tract, uterus, and arteries, uterine endometrium and fat cells. The protein was identified by Western blotting in membrane extracts of these tissues, and the ectodomain antibody immunoprecipitated a protein that was recognised with a P2X4 receptor C terminus antibody. The findings indicate that the P2X4 receptor subunit has a very extensive distribution among mammalian tissues, and this suggests possible new functional roles.