Prolonged nasogastric suction and the administration of magnesium-free parenteral fluids produced magnesium deficiency in a patient in the postoperative period. The symptoms of magnesium deficiency are usually overshadowed by those of the primary disease and diagnosis usually rests on the demonstration of low blood concentrations of the ion. Magnesium replacement must be kept in mind when treating patients with intravenous fluids for prolonged periods.