The present paper describes an electron microscopical investigation into the ultrastructure of patellary and femorocondylar articular cartilage from adult guinea pigs, mice and young rats. The following observations were made: 1. The intercellular matrix contains two varieties of collagen fibrils, one being 85 A in diameter and not striated, the other having a diameter of 550 A and presenting striation with a spacing of 580 A. 2. With respect to the nature of the fibrils the intercellular matrix can be divided into three regions: a) the capsule without fibrillar elements, b) the territory where the fibrillar element becomes increasingly abundant and more fully developed as the periphery is approached, and c) interterritorial region with fibrillar elements of constant diameter and uniform striation. 3. From the chondrocytes cytoplasmic processes project, 0.36 microns long and 0.06 microns wide. 4. The chondrocytes possess a well developed system of α-cytomembranes (endoplasmic reticulum) located peripherally in the cytoplasm. 5. Comparatively extensive domains of homogeneous matter are likewise encountered peripherally in the cell. 6. Within a semilunar juxtanuclear region vacuolar formations are encountered measuring 0.37 microns in diameter and exhibiting a light peripheral zone circumscribing a denser centre. 7. The mitochondria are few, rounded or elongated, and contain a small number of short internal membranes. 8. The calcified stratum of articular cartilage is characterized by the presence of degenerated chondrocytes and, especially in interterritorial portions, focal deposits of inorganic matter.