In an attempt to further define the nature of the active metabolite in bone formation, a series of experiments were conducted whereby vitamin D metabolites were administered locally in vivo into the proximal epiphyseal growth plate of the tibiae of rachitic chicks. Local administration of 3 micrograms of 24,25(OH)2D3 in vivo to D-deficient chicks resulted in disappearance of the rachitic lesions in the same leg. Administration of 1 microgram 1,25(OH)2D3 in a similar manner failed to show any sign of healing. Injection of 5 micrograms 25(OH)D3 was followed by recovery from rickets in both the injected right leg and in the vehicle-injected left tibia, although the recovery was more pronounced in the injected leg. Lower doses of 0.3 or 1 microgram 24,25(OH)2D3 failed to reverse the rachitic lesions and induced only minimal recovery. These findings suggest that 24,25(OH)2D3 at the higher doses has a direct local effect on cartilage and bone, while 1,25(OH)2D3 has no such effect in chicks. 25(OH)D3 is probably absorbed from the epiphyses into the blood stream and converted into the active metabolites, which were indeed detected in the blood to exert its systemic effects.