We have found lower serum phosphorus and higher 1,25-dihydroxyvitamin D (1,25(OH)2D) concentrations in black compared to white infants less than 18 months of age, despite similar, and normal, 25-hydroxyvitamin D concentrations. In addition, we have observed a higher bone mineral content (BMC) at one year of age in black compared to white children. The reasons for race differences in serum phosphorus, 1,25(OH)2D concentrations and BMC are currently unknown, but may provide insight as to why there is a lower rate of bone demineralization among black women compared to white women. The overall aim of this study is to determine, in a longitudinal fashion, whether there are race and sex differences in BMC and calcium metabolism from 3 to 18 months of age and whether differences are related to physical activity. Physical activity significantly increases bone mineralization in adults; black infants are known to have advanced gross motor development compared to white infants. We theorize that higher activity levels result in increased osteoblastic activity, as indicated by increased serum osteocalcin concentrations, and diversion of calcium and phosphorus from blood, resulting in an increase in serum parathyroid hormone (PTH), and 1,25(OH)2D. Renal effects of elevated PTH and 1,25(OH)2D, along with increased intestinal absorption of these minerals and continued high turnover of bone, will result in normalization of serum calcium and a decrease in phosphorus concentrations. Infants will be enrolled at 3 months of age and at 6 months they will be randomized, within each race, into either an "exercise" (age-specific large motor activities) or "no-exercise" program (fine motor/cognitive activities). Spontaneous activity will be quantified using monthly 24-hour motion sensor measurements and direct observation every 3 months. The intervention program will be conducted in the child care setting by study personnel. Throughout the first 18 months of age, the following will be obtained: blood samples for mineral and calcitropic hormone measurements; urine samples for estimating calcium and cyclic adenosine 3',5'- monophosphate excretion, and tubular reabsorption of phosphorus; BMC determinations; anthropometric measurements; motor development assessment; and dietary information. This study will provide important information on race differences in infant BMC and calcium metabolism and whether gross motor activity during infancy can increase bone mineralization, theoretically increasing peak bone mass achieved later in life.