The authors report a case of ventricular tachycardia (VT) induced by theophylline toxicity in a patient with ischaemic heart disease complicated by left ventricular failure and chronic asthma. Two episodes of VT were reduced by intravenous injection of magnesium chloride. The aggravating role of a moderate magnesium deficiency is suggested by the synergy of hypomagnesemia and theophylline on intracellular calcium fluxes contributing to an accumulation of intracytoplasmic calcium, a situation which has been shown experimentally to generate delayed after depolarisations. The respective roles of the correction of the metabolic deficiency and of the specific antiarrhythmic action related to the calcium blocking effect of magnesium salts at cellular level are discussed.