The alteration in bone metabolism with increasing age was investigated in the femoral diaphysis of male rats. Calcium content was highest in the bone from 3-week-old rats (491±13 mg/g bone ash), falling gradually with aged to 357±7 and 306±9 mg/g bone ash in 28- and 52-week-old rats, respectively. Bone zinc content increased until rats were 3 weeks of age, and thereafter remained constant. Deoxyribonucleic acid (DNA) content was highest in the bone from 1-week-old rats, and it decreased markedly with increasing age. Alkaline phosphatase and acid phoshatase activities increased up to 3 weeks, then subsequently declined with age. Thus, the retardation of bone metabolism was induced by ageing. When zinc sulfate (5.0, 10.0 and 20.0 mg Zn/kg body weight) was administered orally for 3 d to 28-week-old rats, alkaline phosphatase activity and calcium content in the femoral diaphysis was elevated markedly by all doses. The oral administration of vitamin D₃ (2.0 and 20 μg/kg) for 3 d in 28-week-old rats did not produce an appreciable increase in bone alkaline phosphatase activity or calcium content, while 1, 25-dihydroxyvitamin D₃ (1.5 μg/kg) caused a significant increase in those biochemical indices. These results indicate that zinc and 1, 25-dihydroxyvitamin D₃ play a role as activators in bone metabolism of ageing rats.