A simple procedure has been developed for preparation of animal tissues and diet mixtures for Mg and Ca analysis by atomic absorption spectroscopy. This procedure involves wet ashing in warm 30% hydrogen peroxide and extraction of metals, from the dried ash, into cold, dilute nitric acid. Less than 200 mg of tissue or feed are required. Measurements obtained by this method for hamster and rat myocardium give values similar to those found previously by more cumbersome methods. Concentrations of Mg and Ca in hamster serum, bone, kidney, and liver are also reported. Recovery of magnesium from bone and heart tissue was 101% and 102%, respectively. For calcium the respective values were 99% and 98%. This procedure is simple to carry out, does not generate corrosive fumes or require expensive equipment. It can be conveniently used to prepare large numbers of samples for atomic absorption spectrophotometry.