Two groups of patients with multifocal atrial tachycardia (MAT) were treated with the intramuscular and continuous intravenous magnesium sulphate regimens used in pre-eclampsia. Both routes of administration were successful in causing reversion to sinus rhythm but the intramuscular regimen, by attaining a higher and more sustained serum magnesium concentration, converted the arrhythmia to normal sinus rhythm in a shorter period of time (1-2 hours) than the intravenous regimen (4-8 hours). Magnesium-sparing diuretics should be used in the treatment of patients with chronic obstructive pulmonary disease and congestive heart failure, which are both conditions associated with magnesium deficiency and MAT.