Many studies have shown that magnesium ions play an important role in the regulation of cellular and physiological metabolism in most systems within the organism. Moreover, epidemiological studies indicate that the amounts of magnesium consumed by considerable percentages of the population are below the recommended intakes, and that hypomagnesemia is present in 10% to 15% of the western population. However, the clinical manifestations of magnesium deficiency are difficult to define because depletion of this cation is associated with considerable abnormalities in the metabolism of many elements and enzymes. If prolonged, insufficient magnesium intake may be responsible for symptoms attributed to other causes, or whose causes are unknown. Many experimental models in laboratory animals have been used to investigate the alterations caused by magnesium deficiency, and these models can be divided into two main types: acute deficiency lasting a few days, and chronic deficiency in which a diet deficient in magnesium is given during a period of several weeks or for up to 1 year. Chronic studies aim to mimic the situation observed in western countries, that is, moderately low intakes o f magnesium sustained over long periods. In this review we will summarize the results reported from long-term studies in experimental animals.