It is a well-known fact that longitudinal bone growth is affected especially by hypophysis among other various endocrine organs. Using young rats, changes in body weight and in length of the tibia were studied after removal of hypophysis, and alterations occurring in the proximal epiphyseal cartilage plate of the tibia playing an important role in longitudinal bone growth were examined under light and electron microscopes, with special emphasis on the proliferative zone. Alterations of ATP-ase activity of ruthenium red affinity were also examined electron microscopically. Further, quantitative analyses of the alterations of cell organelles were performed using low power electron micrographs. 1) Following hypophysectomy, immediate suppression in weight gain occured, and little or no gain was seen from 14 to 28 days after removal of the gland with plateau in body weight. Longitudinal bone growth of tibia was also suppressed, and little or no growth was seen from the 14th day afterward as was the case for body weight. 2) Changes revealed by light microscopic examination included reduced width of epiphyseal cartilage plate, decreased number and disturbed arrangement of chondrocytes, rapid transition from proliferative zone to degenerative zone, and disturbed formation of the subchondral trabecula. 3) Electron microscopic examination revealed poor development of rough surfaced endoplasmic reticulum in the cell of proliferative zone, Golgi apparatus ocnsisting of lamellae and a relatively large numbers of vesicles and pronounced decrease in vacuoles. In addition, accumulation of lipid droplets and glycogen granules were seen together with mitochondria exhibited swelling and elevation of electron density in their matrices. ATP-ase activity was decreased. In the cartilage matrix of the proliferative zone, collagen fibrils and ruthenium red positive granules were decreased in number. 4) Rough surfaced endoplasmic reticulum, Golgi apparatus, and cytoplasmic vacuoles were treated quantitatively by dividing the proliferative zone into two parts of upper and lower layers. The data showed marked decrease in the every item examined after hypophysectomy. 5) These results indicate that hypophysectomy causes a decrease in intracellular metabolism of chondrocytes in the proliferative zone with a reduced function of matrix formation. The suppression of longitudinal bone growth after hypophysectomy appears to occur under an intimate correlation with a decrease in matrix formation by the chondrocytes of proliferative zone and inhibition of proliferation of the cells in this zone.