Serum magnesium estimation was done in 19 children who had heart failure of varied etiology. Five of nine toxic patients and three of 10 non-toxic ones had magnesium deficiency (serum magnesium < 1.5 mEq. per liter_. Mean serum magnesium level was significantly lowered (P < 0.01) in 19 children and it was further lowered in nine toxic patients (P < 0.001) as well as in eight hypomagnesemic patients (P < 0.001) than in healthy control subjects. Mean serum digoxin level in toxic patients was significantly higher than in nontoxic ones (P < 0.05). In three cases magnesium sulfate was successfully used for the management of cardiac arrhythmias.