The present study was undertaken to evaluate the effects of 1 alpha-hydroxyvitamin D3 and 24R,25-dihydroxyvitamin D3 on bone remodeling in dogs with osteomalacia induced by vitamin D depletion. To assess the rates of skeletal remodeling, intravital tetracycline labeling and morphometry of surface pattern were employed. Either vitamin D3 derivative accelerated the appositional growth rate, increased the percentage of osteoid seams labeled, and decreased the number, width, and perimeter of new osteoid seams. But the derivatives differed in bone resorbing activity: 1 alpha-hydroxyvitamin D3 increased the number and perimeter of resorption sites whereas 24R,25-dihydroxyvitamin D3 decreased them. Thus the results show that the former is a better bone remodeler while the latter may be useful in treating osteoporosis.