Modifications of the pyroantimonate technique were used to localize intracellular sites of bound cations in the pars distalis of normal and hypothyroid rats and in rats with increased levels of plasma calcium. In the normal animal, cations were localized within most intracellular organelles and sometimes on the membranes involved in exocytosis. The amount of bound cations within pituitary intracellular organelles was considerably augmented in rats injected intravenously with calcium chloride. However, in the thyroidectomy cells of hypothyroid rats, the amount of cation precipitate appeared to be selectively increased in the dilated rough endoplasmic reticulum (RER). The calcium chelator, EGTA, and X-ray microprobe analysis revealed the presence of calcium in cation deposits.