Two premature newborn infants developed extreme magnesium toxicity while receiving total parenteral nutrition (TPN) infusion. Both patients exhibited acute hypotonia, apnea, hypotension, and refractory bradycardia mimicking septic shock syndrome. The complete blood count was normal, and blood cultures were negative. Serum magnesium concentration in 1 patient was 43.1 mEq/L and in the other patient was 45 mEq/L (normal values for serum magnesium being 1.6-2.1 mEq/L). Hypermagnesemia resulted from malfunction of an automated TPN mixing device. Unexplained sudden onset of apnea, refractory bradycardia, and hypotension should raise suspicions of hypermagnesemia, a reversible condition if identified and treated early.