The effects of feeding with a purified magnesium-deficient diet (--Mg, 7-8 mg Mg/kg) on horse foal blood serum and tissue concentrations of Mg, calcium and phosphorus were studied, and the results compared with histopathological findings. 2. Serum concentrations of Ca and P were unaffected by feeding with the --Mg diet, whereas serum Mg concentrations decreased from a mean initial (day o) concentration of 0.78 mmol/l to 0.53 mmol/l 7 d after foals were placed on the -Mg diet, and then continued to decrease at a slower rate. 3. Aorta concentrations of Ca and P, but not Mg, were positively correlated with the period of time foals were given the -Mg diet, verifying histopathological findings. Results for both aorta Ca and P analyses and histopathological studies indicated that mineralization of the aorta began approximately 30-35 d after foals were placed on the -Mg diet. 4. Feeding with the -Mg diet had no significant, analytically detectable effect on brain, liver, kidney, lung, spleen, skeletal or cardiac muscle concentrations of Ca, P or Mg, although microscopic evidence of mineralization was seen in some of these tissues from foals given the Mg diet for 71-180 d. 5. A significant negative correlation was found between bone ash concentrations of Mg (rib, metacarpus and metatarsus) and the length of time foals were fed on the Mg diet. Bone ash concentrations of Ca and P were, however, unchanged. 6. Low serum Mg values and negative correlations between the bone ash concentration of Mg and the period of time foals were fed on the -Mg diet supplemented with 390 mg. Mg as MgO/kg were interpreted as suggesting that either this level of Mg supplementation is marginal for the growing foal, or that the Mg in MgO is not readily available to the growing foal.
