Sixteen implants of fetal growth cartilage of New Zealand white rabbits of both sexes were implanted intramuscularly into an adult animal of the opposite sex. Endochondral ossification was recognized in 11 of the 16 implants. The sex of the osteoblasts and chondrocytes was determined by counting the sex chromatin body in five areas of abundant endochondral ossification. The bone grown adjacent to the implant was predominantly of recipient type in all of the specimens (p less than 0.01). Most of the chondrocytes gradually lost stainability to Feulgen-Schiff stain as they matured, and only a few cells were stained in the calcified zone, indicating the deterioration of nuclear DNA. Thus, the majority of the growth plate chondrocytes are considered to die and be replaced by osteoblasts of a different origin.