The initial nutritional status of experimental animals can influence their response to subsequent dietary regimens. In the present study, we determined the variations in minerals in diet NIH-31, a breeding colony stock diet, and in tissues of weanling rats nursed by dams fed this diet. Inductively coupled plasma-atomic emission spectrometry (ICP-AES) was used to determine nine elements (Ca, Cu, Fe, K, Mg, Mn, Na, P and Zn) in diet and in liver, kidney, spleen, duodenum and femur from 22- to 26-day-old rats. Wet digestions were performed in mixtures of nitric, perchloric, and sulfuric acids (diets and soft tissues) or nitric and perchloric acids (femur). Solution concentrations ranged from less than 25 ng/ml for the trace elements to greater than 100 micrograms/ml for the major elements. Large variations in mineral content were found between batches of commercially prepared NIH-31 diet; relative amounts of Cu, Fe, Mn and Zn varied markedly. Significant differences in concentrations of major and trace minerals in liver, kidney, spleen and duodenal tissue were found among groups of weanling rats obtained from the same supplier at different times. Mn was readily quantitated in all tissues except spleen, where it was below detection limits. The precision obtained with the ICP-AES methodology has significant advantages for establishing variations in tissue mineral levels.