Using female rats from 2 to 64 weeks old, the changes in bone gamma-carboxyglutamic acid-containing protein (BGP) levels in bone and serum were studied in relation to the changes in calcium metabolism during aging. Intestinal calcium transport, serum phosphorus level, bone origin serum alkaline phosphatase activity and serum BGP content were high in rapidly growing animals and then decreased with aging. Serum BGP content correlated well to bone origin serum alkaline phosphatase activity. Bone density, bone ash content and bone BGP content increased during aging. Bone BGP content was correlated to bone density which indicates the level of bone calcification. Moreover, the effect of castration on calcium metabolism and bone and serum BGP contents were observed in young (10 weeks old) and aged (40 weeks old) female rat at 12 and 24 weeks after operation. Intestinal calcium transport in ovariectomized rat was significantly lower than in sham operated rat. Serum phosphorus level and serum BGP content were increased in castrated female rat. However, serum calcium level and bone origin serum alkaline phosphatase activity did not show a significant change in castrated female rat. Bone density was significantly decreased in aged rat at 24 weeks after operation. During aging or castration, serum BGP content changed more than bone BGP content. The determination of change in serum BGP levels in various disorders of calcium metabolism would be very informative in future study.