The electrocardiogram was recorded and serum and bulk myocardial electrolytes were determined in male Sprague Dawley rats, subjected to dietary magnesium deficiency for various periods, to assess the time course of development and cessation of the enhanced arrhythmogenic action of isoproterenol (150 micrograms/kg, subcutaneously) and to establish possible relationships between electrolyte changes and severe ventricular dysrhythmias. Ventricular fibrillation occurred within 60 min following isoproterenol injection in 25, 25, 62.5, 50, and 62.5% of rats on magnesium deficient diet for 4, 7, 11, 15, and 19 days (N = 8), respectively, and resulted in death in most animals (83%). Reintroduction of normal chow following a 30-day period on magnesium-deficient diet normalized serum magnesium (from 1.42 +/−0.23 to 1.90 +/−0.08 mEq/liter, mean +/−SD) but did not significantly reduce the incidence of ventricular fibrillation. Magnesium deficiency did not produce statistically significant alterations in bulk myocardial content of sodium, potassium, magnesium, and calcium. However, sodium was elevated and potassium diminished in hearts from rats that died in ventricular fibrillation, but not in those that had recovered. Magnesium-deficient rats sacrificed 30 min after isoproterenol injection, that is before the occurrence of ventricular fibrillation, exhibited hypomagnesemia and hypokalemia as well as elevated sodium and diminished potassium and magnesium in the myocardium. In contrast, rats on Purina Chow exhibited hypermagnesemia, but also showed hypokalemia and diminished cardiac potassium. The results indicate that magnesium deficiency enhances the arrhythmogenic propensity of isoproterenol and that the development of ventricular fibrillation is preceded by serum and myocardial electrolyte alterations.