The effects of a high dose of ascorbic acid superimposed on a low magnesium diet were studied for the first time. Young male guinea pigs were fed for six weeks two diets containing 3000 or 600 ppm magnesium: half in each group was supplemented with a daily oral dose of either 3 mg or 100 mg ascorbic acid per 100 g body weight. No treatment effects were found in serum copper and ceruloplasmin, spleen copper, bone calcium, kidney magnesium, and brain calcium and magnesium contents. Both bone copper and brain ascorbic acid contents of the group fed the normal ascorbic acid/low magnesium diet were lower (p less than 0.01) than the combined means of the other three groups; the high ascorbic acid/low magnesium treatment resulted in normalization of bone copper and brain ascorbic acid levels. Irrespective of ascorbic acid level, the low magnesium diet decreased the bone magnesium and increased the kidney calcium contents (p less than 0.01); this effect on kidney was nearly doubled by the high ascorbic acid intake (p less than 0.01). The results indicated that the main effects were due to the magnesium deficit.