Few measurements of magnesium (Mg) kinetics have been performed, especially in children and adolescents. Simultaneous Mg and calcium (Ca) kinetics following intravenous administration of42Ca and25Mg were measured in 22 children (10 boys and 12 girls) 9–14 years of age. Kinetic values, including the body pool masses (MgComp and CaComp for the Mg and Ca compartment masses, respectively) and bone/tissue (hard and soft tissue) Ca and Mg deposition rates (Vo+Ca and Vo+Mg, respectively) were calculated using the Simulation, Analysis, and Modeling (SAAM) program. No significant differences were found between males and females (p > 0.3 for each comparison) for pool masses or deposition rates. Vo+Ca and Vo+Mg were highly correlated (r = 0.78, p < 0.001). Vo+Mg, but not Vo+Ca, was closely correlated with body weight (r = 0.55, p < 0.01 and r = 0.16, p = 0.47, respectively). Similarly, MgComp was more closely correlated with body weight (r = 0.76, p < 0.01) than CaComp (r = 0.37, p = 0.10). Neither Vo+Ca nor CaComp increased significantly over the age range studied. However, Vo+Mg and MgComp were significantly positively correlated with age. These findings demonstrate differences in regulation of Ca versus Mg kinetics by young adolescents. These differences may be related to the greater utilization of Mg for muscle and soft tissue growth. The close relationship between Vo+Mg and body weight suggests that the current practice of relating Mg intake requirements to body weight, as well as age, is appropriate for young adolescents.