The ultrastructure of the upper tibial epiphyseal plate during the development of acute, experimental scurvy in the guinea pig was investigated. The cartilage was examined after the animals had been maintained, under standardized conditions, on a scorbutogenic diet for 8, 15, or 23 days. Progressive fine structural changes were observed in the experimental animals. Within the chondrocytes, the cisternae of the endoplasmic reticulum were rounded, non-continuous, and reduced in extensiveness. There was a decreased number of ribosomes on the cisternal membranes and, concomitantly, an increased amount of free ribosomes in the ground cytoplasm. The secretory vacuoles of the Golgi complex were few. Lysosomes were more frequent than in the controls. Signs of autophagocytosis were often observed. In the ground cytoplasm lipid droplets occurred somewhat more often than in the control animals. In the intercellular substance the amount of collagen fibrils and proteoglycan particles was decreased. Electron-dense matrix vesicles were numerous in the longitudinal septa. The nature of these changes is discussed with regard to the biochemical alterations that have been observed in this disorder.