1. Mitochondria of the adrenal gland, corpus luteum, theca interna and granulosa of the graafian follicle, interstitial cells of the testis, and placenta of the rat have been studied with the electron microscope.
2. In most sites of steroid secretion, the internal structure of the mitochondria is in the form of tubular reflections of the internal mitochondrial membrane, rather than plate-like cristae.
3. The mitochondria of the adrenal gland have a highly variable size, even within a single cell; they may reach 2 to 3 µ in diameter, and are nearly filled with tubules.
4. It is suggested that while mitochondria in many places have a tubular component, those in the adrenal gland are highly specialized by virtue of their giant size and their internal structure.