We exploit the optical and spatial features of subwavelength nanostructures to examine individual receptors on the plasma membrane of living cells. Receptors were sequestered in portions of the membrane projected into zero-mode waveguides. Using single-step photobleaching of green fluorescent protein incorporated into individual subunits, the resulting spatial isolation was used to measure subunit stoichiometry in α4β4 and α4β2 nicotinic acetylcholine and P2X2 ATP receptors. We also show that nicotine and cytisine have differential effects on α4β2 stoichiometry.