The use of an aqueous solution of pyroantimonate as a specific histochemical marker for intracellular sodium is not supported by this study since, at physiologic levels, potassium as well as sodium appears to be precipitated by such a pyroantimonate solution. Furthermore, a low pH and the addition of OsO4 as a fixative compound appear to favor the production of nonspecific electron-dense precipitates with this technique in vitro and in vivo. The addition of phosphate anion to a solution of pyroantimonate seems to utilize the sites at which many cations bind to pyroantimonate, thus elevating the concentration of cation needed to produce a precipitate. Artificial elevation of sodium above physiologic levels results in a precipitate with phosphate-buffered pyroantimonate and probably represents a valid demonstration of sodium since no other cation is present in sufficient concentration to produce an electron-dense deposit with this medium.