The present work was performed in order to study the chemical composition of the mitochondrial granules occurring in the main and the Sachs' electric organs of Electrophorus electricus L. using cytochemical techniques. The potassium pyroantimonate technique gave positive reaction in the granules, demonstrating its calcium content. EGTA treatment of ultrathin sections, a calcium specific chelating agent, dissolved the precipitate. For detection of organic substances, the following techniques were employed: 1) the thiosemicarbazide-silver proteinate technique, showing silver proteinate precipitates in the granules, suggesting the presence of glycidic substances; 2) the zinc iodide-osmium tetroxide (ZIO) technique with marked deposition of reaction product in the granules, probably indicating the presence of lipids and/or precursor proteins; 3) with simple fixation by glutaraldehyde the granules did not show up, they were conspicuous however when OsO4 was used as a fixative, alone or after glutaraldehyde fixation; this suggests a phospholipid content; 4) the use of imidazole buffered osmium tetroxide confirmed this composition and indicates that cholesterol and phospholipids are present in the granules. The results obtained so far suggest that the mitochondrial granules of the electrocytes have a complex chemical composition that includes organic and inorganic substances, as calcium, lipids, sugar, and proteins.