Studies were conducted to determine the effect of age on magnesium requirement and whether high dietary magnesium during the first 7 days of life would protect against subsequent magnesium deficiency. Day-old Japanese quail were fed a casein-gelatin diet containing either 300 (requirement) or 600 ppm Mg. Beginning on day 7, half of the birds in each group received either 160 (deficient) or 300 ppm Mg. In birds fed the requirement level during week 1 followed by the deficient diet, hemoglobin decreased on day 10, reaching the lowest value on day 14. Other adverse effects included mortality; slightly reduced growth rate; elevated iron in the liver, spleen, and tibia; decreased tibia magnesium; and increased spleen weight. Higher dietary magnesium (600 ppm) during week 1 prevented deficiency when 160 ppm Mg was fed in week 2; however, tibia magnesium was lower than that in birds fed 300 ppm Mg throughout. The protection may be related to reutilization of skeletal magnesium, which was elevated on day 7. In this and another experiment, birds initially fed 300 ppm Mg followed by 160 ppm (7-28 days) or 125 ppm Mg (14-28 days) had normal hemoglobin values at 21 and 28 days. These experiments showed that magnesium requirement decreased as quail aged (up to 4 weeks) and that a high magnesium diet (600 ppm) fed through days 0-7 protected against subsequent dietary inadequacy.