Chondrocytes isolated from auricular cartilage of 7-day-old rabbits were grown in vitro until the onset of phase III, occurring after 10-14 population doublings (PD). The size of cells and their dry mass were measured at various PD levels. These data were compared with results of analogous measurements of chondrocytes freshly isolated from 28-day-old rabbits. Both in vivo, during cartilage growth, and in vitro, some of the chondrocytes increased considerably in size and acquired two nuclei. Chondrocytes cultured in vitro for 4 population doublings were still capable of depositing elastic fibers in culture and forming cartilage after intramuscular transplantation. After longer periods of cultivation the ability of cells to produce a cartilage matrix declined. It is suggested that the auricular chondrocytes may represent a convenient model for comparative studies of cell aging in culture and in vivo, owing to the simplicity of matching senescent cells arising in both these situations.