Two distinct classes of mitochondria are described in the normal adrenal cortex of the Sprague Dawley CFY rat. Polyaminar mitochondria were frequently observed in the zona fasciculata and zona reticularis, particularly after ACTH stimulation of the cortex resulting from cold-stress exposure. It is uncertain whether such organelles are degenerating forms, or whether they have a specific functional role related to steroidogenesis in the normal cortical cell. In both normal and stressed adrenal cortices, protrusions of the outer membrane of mitochondria were evident, and were often seen penetrating lipid droplets. It is suggested that these protrusions may have some significance in the transport of cholesterol from the lipid droplet to the inner mitochondrial memrane 'desmolase complex', thus facilitating side-chain cleavage of cholesterol to pregnenolone.