We treated a case of iatrogenic hypermagnesemia that clinically mimicked a central brain-stem herniation syndrome. Hypermagnesemia (magnesium level, 9.85 mmol/L [24 mg/dL]) can cause parasympathetic blockade, inducing fixed and dilated pupils, in addition to neuromuscular blockade. Extreme hypermagnesemia can therefore mimic a midbrain syndrome and cause a pseudocomatose state.