The cytoplasmic components of chondrocytes from the various zones of the rat costochondral junction, grown in vitro, were quantitated from electron micrographs using a point-counting technique, and the results were compared statistically with the cytoplasmic components of chondrocytes in the corresponding zones of the junction in vivo. Detectable changes occurred in vitro as early as after six hours of culture. The major changes that occurred in vitro were: an increase in the amount of lipid bodies in chondrocytes in the hypertrophic zone and in the bottom portion of the proliferative zone, failure of the chondrocytes in the bottom portion of the hypertrophic zone to increase in size, retention of intactness of the cell membrane of chondrocytes at the base of the hypertrophic zone, a decrease in the number of mitochondria in chondrocytes in the hypertrophic zone and in the middle and bottom portions of the proliferative zone, and a decrease in the number of holes in the cytoplasm of chondrocytes in the bottom one-half of the hypertrophic zone. In vitro hypertrophic chondrocytes of the growth plate demonstrated cellular changes associated with aging, while in vito the hypertrophic chondrocytes demonstrated changes associated with degeneration. The final result in both cases was cell death.