One of the consequences of uncontrolled diabetes mellitus (DM) is an increased urinary loss of magnesium. Some of the factors determining this electrolyte imbalance were studied in growing rats with experimentally induced DM. Two sets of experiments were performed in which dietary magnesium intake was the variable. One group each of diabetic (DB) and control (C) animals was fed a complete diet including magnesium ad libitum; the other set was given the same diet as the first two groups but the magnesium was provided daily through a stomach tube in amounts equal to those eaten by non-DB rats of the same size. All DB animals had significantly higher than normal urinary excretion of glucose, magnesium, calcium, and phosphorus. There was a positive correlation between urinary glucose and magnesium excretion. DB animals fed magnesium ad libitum became hypomagnesemic, but their bone magnesium content was increased over that of C rats. Plasma magnesium correlated negatively with both glucose and cholesterol. However, alterations in plasma cholesterol induced in C rats by dietary means had no effect on magnesium levels. When magnesium intake was restricted to the physiologic requirements of C animals, hypomagnesemia was more pronounced and occurred with concomitant depletion of magnesium in bone, suggesting possible risk of electrolyte imbalance in uncontrolled DM. The results indicate that hypomagnesemia without osseous tissue magnesium depletion may occur in experimental DM. Hyperphagia in DB animals fed ad libitum can prevent bone magnesium depletion, but a “normal” intake that does not compensate for losses may be conducive to a marked deficit in the intracellular pool of magnesium.