The recommended dietary allowance for calcium for the female adolescent is an extrapolated figure. Very little information exists on normal calcium metabolism in this age group. Meeting calcium requirements for maximal bone growth may be the most crucial in this age group to lower the risk of developing osteoporosis in later years. The proposed study will use dual isotopic tracer techniques combined with a three week metabolic study in 10 adolescent girls aged 12-14 and 10 young women aged 20-30. Major hypotheses to be tested include: 1) Skeletal requirements of adolescent girls are greater than in adult women as reflected by a) increased bone accretion, b) greater calcium retention, and c) reduced calcium urinary losses. 2) Calcium fractional absorption and endogenous fecal losses are not greater in adolescent girls than in adults. 3) Bone calcium turnover is grater in adolescent than adult females. Stable isotopes of calcium will be administered orally and by intravenous injection after a one week equilibrium period on a controlled diet contain- ing 1200 mg Ca/day. Total calcium will be analyzed by atomic absorption mass spectrometry and stable isotopes will be analyzed by high resolution fast atom bombardment mass spectroscopy. Analysis of complete urine and fecal collections and periodic blood samples will provide data for multi- compartmental and stochastic analysis of calcium metabolism. Correlations with hormone levels and biochemical markers of bone turnover will also be made.