Magnesium in plasma, erythrocytes, and cerebrospinal fluid (CSF) was measured immediately after hospital admission in 9 patients with delirium tremens (DT) and 11 patients with impending DT. Blood samples were taken daily during the acute state; a second lumbar puncture was performed when the patient's condition had improved. Plasma magnesium was low in patients with DT during the first days of the acute state and then spontaneously normalized. Normal plasma magnesium was consistently seen among patients with impending DT. Magnesium in erythrocytes and CSF was normal in both diagnostic categories. Patients with a high blood-alcohol concentration (BAC) at admission had a decreasing plasma magnesium, patients with a low BAC had a moderately increasing plasma magnesium, and patients with a BAC at nil had a more marked increase in plasma toms or with their duration. This finding, combined with the normal CSF magnesium and the lack of correlation between plasma and CSF magnesium, indicates that disturbances in magnesium metabolism do not play a role in the etiology or pathogenesis of DT; but it may be that disturbances in magnesium metabolism contribute to the development of alcoholic encephalopathy.