We studied changes in the mineral composition of milk of lactating Swiss-Webster mice and the relationship of those changes to mineral metabolism of suckling mouse pups. Concentrations of Zn, Cu, Mg and Ca were analyzed in maternal and neonatal tissues and in milk; Zn metabolism was studied using 65Zn. Although Cu, Ca and Mg concentrations in milk declined during the first 2 d of lactation, only the concentration of Zn decreased progressively throughout 30 d. Various pup tissues were characterized by developmental changes in concentrations of some elements. Turnover of Zn in neonatal tissues was studied by radiolabeling litters in utero and fostering them at birth to nonradiolabeled lactating dams. The turnover of whole-body 65Zn in suckling mice decreased during development, and, at 20 d of age, the biological half-life of 65Zn in the neonate was approximately the same as for a nonpregnant, nonlactating adult female (20 d). The decreased turnover of 65Zn in lactating dams that characterized progressive lactation and was reflected in the Zn concentration of milk is in agreement with changes in whole-body 65Zn turnover observed for the suckling mouse. These findings demonstrate that the metabolism of the suckling neonate is directly related to longitudinal changes in the composition of maternal milk.