To study the in vivo effects on bone formation in standardized bone growth and remodelling systems, a moderate supplement of vitamin D metabolites was given adolescent rats on a standard diet. Comparison with other hormones previously shown to stimulate the bone formation in these systems is thereby facilitated. Female rats (10 animals per group) were given one daily subcutaneous injection of 1.5 µg/ kg 25-OHD3 or 0.04 µg/kg 1,25-(OH)2D3 for 20 days (75–94 days of age). Controls were given 0.5 ml solvent. Tetracycline was used as intravital marker to determine the longitudinal bone growth of proximal tibia as well as the apposition and resorption of cortical bone in the femur diaphysis. The osteoclasts were counted in the metaphysis and diaphysis of the ribs. Vitamin 1,25-(OH)2D3 resulted in retardation (p$0.05) of the accumulated longitudinal bone growth. A similar but insignificant tendency was found for 25-OHD3. The body weight and the periosteal and endosteal bone formation at various levels along the femoral shaft were unchanged compared to the controls. The total cortical width was somewhat lower in the distal femur for 1,25-(OH)2D3. In the proximal femur 25-OHD3 resulted in a somewhat thicker cortical bone. Most levels along the femur, however, showed no differences. The osteoclast count showed an increased number (p $ 0.05) in the cortex of the rib in the rats given 1,25-(OH)2D3. Compared to growth hormone, which is the major bone growth stimulating agent, the influences on bone formation of the vitamin D metabolites were minor and predominantly negat