Abstract
1. Adrenocortical mitochondria contain filamentous or saccular internal membranes and are probably the sites of lipide elaboration or accumulation. They are numerous in the stimulated gland and scanty after hypophysectomy. There is evidence that they are expendable: light cells contain numbers of irregularly vacuolated and broken mitochondria with disorganized internal membranes. 2. Openings have been observed in some mitochondrial limiting membranes, particularly in the stimulated hamster and rat adrenal cortices, but to a lesser extent in the unstimulated glands. 3. Similarities between mitochondrial and Golgi membranes suggest a possible mode of mitochondrial formation. 4. These findings have been interrelated and discussed.