We assessed the change of bone mineral density (BMD) in lactating beagles with dual energy X-ray absorptiometry (DXA) and the preventive effect of 1α-hydroxyvitamin D₃ (1α(OH)D₃) on the BMD. Beagles, two to five years old, were used for detecting the time course change of BMD. Since the coefficient of variation (CV(%)) on detecting lumber vertebral (L2-L4) and tibial BMD by DXA was about 0.5%, DXA was useful to detect the change of BMD in beagles. There was a marked decrease in vertebral BMD during lactational period in the control group. The BMD levels after weaning were found to reverse to the initial level at mating. The same tendency was observed in tibial BMD as vertebral BMD, though the BMD changes were not marked. Beagles were administered at a dose of 0.1 μg/kg of 1α(OH)D₃ three times in a week, and it was found to suppress the decrease in vertebral BMD during the breast feeding period. Also, the administration of 1α(OH)D₃ promoted the prevention of decreased BMD during lactation both in vertebrae and tibiae. Significant effects of 1α(OH)D₃ administration on tibial BMD were not observed. No adverse effects, such as hypercalcemia and hypercalciuria, were observed during the experimental period. Therefore, DXA was useful for detecting the changes of BMD in lactating beagles and the change of BMD was marked in lumber vertebrae, which are rich in trabecular bone. The preventive effect of 1α(OH)D₃ on the decrease of BMD during the lactation period was observed in beagles.