Pregnancy in the setting of renal failure has higher rates of adverse events necessitating increased monitoring and treatment. Pregnant women with end-stage renal disease have higher rates of hypertension, and 50% of pregnancies are complicated by preeclampsia. We describe the case of a 32-year-old parturient with end-stage renal disease on hemodialysis with superimposed preeclampsia who developed clinically significant hyperkalemia with electrocardiographic changes after magnesium infusion. The magnesium infusion was stopped, and the patient underwent emergent hemodialysis with subsequent improvement. Hyperkalemia caused by magnesium infusion is a rare and not very well-understood phenomenon.