A study with human volunteers was conducted to test the hypothesis that naturally occurring inadequate intakes of magnesium induce negative magnesium balance and undesirable changes in calcium metabolism variables, and that these changes are influenced by dietary boron. Diets composed of ordinary Western foods providing approximately 118 and 318 mg Mg/d and approximately 0.25 and 3.25 mg B/d were fed in a double-blind Latin square design to 13 healthy, post menopausal Caucasian women (aged 50-78 years) living in a metabolic unit. Magnesium balance, which was positive when dietary magnesium was 318 mg/d, became negative when dietary magnesium was 118 mg/d. Magnesium deprivation decreased urinary calcium excretion, and significantly increased calcium balance when balance data analyzed came from all collections during the 42-day periods. Urinary phosphorus excretion was increased, but fecal phosphorus excretion was decreased, thus phosphorus balance was not significantly affected by magnesium deprivation. Magnesium deprivation did not affect manganese or zinc balance. The balance data indicated that 700 mg of calcium, 1.0 mg of manganese, and 10 mg of zinc were adequate for post menopausal women. Magnesium deprivation increased serum 25-hydroxycholecalciferol and decreased serum total cholesterol concentrations. Boron deprivation increased but magnesium deprivation decreased urinary potassium excretion. Boron supplementation decreased serum 17beta-estradiol and progesterone when dietary magnesium was low. The dietary treatments did not affect serum calcitonin, parathyroid hormone, osteocalcin or alkaline phosphatase concentrations. One woman placed on consecutive magnesium-low dietary periods exhibited heart ventricular ectopy after consuming the magnesium-low diet for 72 days; the ectopy disappeared upon consuming the magnesium-adequate diet. The findings indicated that consuming an ordinary diet deficient in magnesium, resulting in negative magnesium balance, can affect calcium, potassium, and cholesterol metabolism. Dietary boron did not have an obvious effect on the response to magnesium deprivation.