The longitudinal growth of bone depends on the activities of individual chondrocytes of the growth plate. Each chondrocyte remains in a fixed location throughout its life, and there accomplishes all of its functions. Although a cell may perform several or all of its activities simultaneously, one of these will usually predominate during a particular phase of its life. The two most prominent stages are those of cellular proliferation and hypertrophy (including the mineralization of matrix) before the resorption of tissue during vascular invasion. By applying recently developed stereological procedures and improved methods for the fixation of cartilage, we compared cellular shape modulation, various ultrastructural parameters (surface areas or volumes of endoplasmic reticulum, Golgi membranes, and mitochondria), the production of matrix, and cellular turnover for proliferating and hypertrophic chondrocytes within the proximal tibial growth plate of the rat. By the late hypertrophic stage, fourfold and tenfold increases in the mean cellular height and volume, respectively, and a threefold increase in the mean volume of the matrix per cell were achieved. The high metabolic activity of hypertrophic cells was reflected by a twofold to fivefold increase in the mean cellular surface area of rough endoplasmic reticulum, the Golgi membranes, and the mean cellular mitochondrial volume. Rates of longitudinal growth were determined by fluorochrome labeling and incident-light fluorescence microscopy. Using these values and the stereological estimators describing cellular height, the rates of cellular turnover were calculated. The rapid progression of the vascular invasion front was found to eliminate, for each column of cells, one chondrocyte every three hours; that is, eight cells a day. The maintenance of a steady-state structure for growth-plate cartilage in rats in a steady state of growth thus necessitates efficient compensation for these losses, which is achieved by a high rate of cellular proliferation and rapid hypertrophy.