An automated fluorometric method is described for the continuous determination of magnesium in whole-blood. Hypermagnesemia produced by withdrawal of blood, asphyxia, infusions of catecholamines or poisoning with potassium cannot be prevented by reserpine, phentolamine or Kö 592. Magnesium normalizes after reinfusion of the withdrawn blood, restarting of breathing or after stopping the infusion of KC1; however, levels remain elevated after stopping the infusion of catecholamines. The administration of epinephrine or isoprenaline produces a slight, but not regularly and dose-dependent occurring initial decrease in blood magnesium. Non-toxic amounts of potassium do not alter the blood magnesium.