The dynamics of Ca metabolism in the human infant were determined using stable isotopes of Ca as tracers. Two isotopes, 1 given i.v. and the other orally, were used. Tracer time-dependent dilutions were measured with high accuracy and precision using thermal ionization mass spectrometry. Similar studies were performed in infant rhesus monkeys using radio tracers. Analysis of the isotope dilutions using a compartmental model method showed quite similar results for the infants of both species. These similarities support the validity both of using stable isotopes in human studies, and the use of the rhesus monkey as a model for human Ca dynamics. The classical steady-state compartmental model was inadequate for use in the rapidly growing infant, and the model was modified to account for the explansion of the body fluid Ca compartment during the time of the study. Comparison with adult human Ca metaolic parameters showed that urinary Ca flows are much higher in the adult than in the newborn, relative to other Ca metabolic parameters. Other parameters of infant Ca metabolism are all consistent with the growth and more rapid metabolism usually seen in the young.