Hypermagnesemia (plasma Mg concentration >2 mg/dL [1.1 mmol/L]) is a rare condition reported only in monogastric animals. Horses show signs of sweating and muscle weakness within 4 hr of receiving excessive oral doses of magnesium sulfate administered as a cathartic for treatment of large-intestinal impactions. This is followed by recumbency, tachycardia (120 bpm), and tachypnea (60 breaths/min). Signs subside after treatment with slow IV infusion of calcium gluconate (23% solution). Hypermagnesemia has been reported in cats with renal failure that were receiving IV fluid therapy. As plasma Mg concentrations exceed 2.5 mmol/L, there may be ECG changes with prolongation of the PR interval; at 5 mmol/L, deep tendon reflexes disappear, followed by hypotension and respiratory depression. Cardiac arrest may occur with blood Mg levels >6.0–7.5 mmol/L.