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The parenteral magnesium (Mg) load test has reliably identified rats fed two extremes of dietary Mg, but tests of intermediate levels have not been found. The present study was designed to learn: 1) whether the parenteral Mg load test reflects a series of different dietary levels of Mg, rather than just the extremes; 2) what relationship exists between Mg retention and Mg in plasma and in bone at the different dietary levels of Mg; and 3) how the different dietary levels of Mg affect weight gain. Weanling rats were fed six levels of dietary Mg ranging from 0 to 150 mg/100 g purified diet. A negative relationship was found between retention of the Mg load and the level of dietary Mg. Mg retention decreased linearly with increases in plasma Mg and bone Mg. Weight gain was an insensitive index of dietary Mg. The parenteral Mg load test appears to be an acceptable means of indirectly assessing Mg stores in weanling rats with normal renal and cardiovascular status and normal water balance.