The purpose of this study was to examine the effectiveness of collagen peptide intake on bone metabolism in growing (G) and calcium-deficient mature (M) rats. As for the dosages used, they were amounts equal to the recommended supplements for humans (0.166 g/kg body weight (BW) per day: Coll-1G and Coll-1M groups), 10-fold higher (1.66 g/kg BW per day: Coll-10G and Coll-10M groups), and 100-fold higher (16.6 g/kg BW per day: Coll-100G group). In growing male rats, bone mineral density (BMD) of the femur in the Coll-100G group was significantly higher than that in the other groups after the 4-week experimental period. On the other hand, kidneys in the rats from the Coll-100G group exhibited hypertrophy. To examine the effects of collagen peptide on bone metabolism in a calcium-deficient status, mature female rats were fed a 0.01% Ca diet for 9 weeks and then fed a diet with 0.2% calcium with or without collagen peptide (control, Coll-1M, and Coll-10M groups) or a 0.5% calcium diet (normal Ca) for 8 weeks. BMD of the whole femur in the Coll-10M group was significantly higher than that in the control and Coll-1M groups, and the level was similar to that in the normal Ca group. BMD of the lumbar spine in the Coll-10M group was significantly higher than their baseline value, as well as being significantly higher than that in the control and Coll-1M groups. These results suggest that orally administered collagen peptide may provide beneficial effects on bone metabolism, especially in the calcium-deficient condition, without obvious undesirable effects.