Light microscopic examination of brain tissue prepared with an improved histochemical method (diamino-benzidine intensified Perl's reaction for ferric iron) has revealed that many brain areas concentrate iron, but that the type of cell in which iron accumulates and the degree of accumulation differ and are characteristic for each area. In the present study the histochemical method has been further modified for identifying the distribution of brain iron at the electron microscopic level. Electron microscopy permits the identification of (1) the type of cell concentrating iron, (2) the location of iron with respect to the cell body and processes of neurones and glia, and (3) the cellular organelle(s) in which iron accumulates. The acquisition of such information is requisite for gaining an understanding of the function brain iron.