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Vitamin and mineral supplementation is often prescribed by physicians to meet the additional nutrient requirements of pregnancy. In order to partially ascertain the effectiveness of these prenatal supplements, the retention of calcium and iron was determined in pregnant women consuming supplemented or unsupplemented self-selected diets. The retention of phosphorus and magnesium, minerals not included in the prenatal supplements, was also determined. Seven-day metabolic balance experiments spaced periodically throughout the pregnancy were conducted on 10 healthy pregnant white women. The retention of calcium by the supplemented group was comparable to that of the unsupplemented group, while the retention of iron was more dependent on the magnitude of the iron intake than on its source. Although no supplement contained phosphate, the intake of phosphorus met the recommended allowances for this mineral. Significantly related to the intake of dietary calcium, the adequate phosphorus intakes reflected diets providing adequate calcium. The mean magnesium intake was only 60% of the recently established recommended dietary allowance. Although the calcium and iron salts provided by the prenatal supplements were well utilized, the intakes of phosphorus and magnesium indicate that the reliance on the effectiveness of mineral supplements should not lessen the emphasis by the physician on the importance of a good prenatal diet.