The effect of dietary magnesium deficiency on the pressor and arrhythmogenic responses to epinephrine was investigated in 19 dogs maintained either on a normal diet (11 dogs) or a diet deficient in magnesium (8 dogs). Magnesium-deficient dogs had significantly lower serum magnesium levels than the control dogs on a normal diet. Magnesium-deficient dogs showed an increased pressor sensitivity to epinephrine as determined by the dose of epinephrine required to cause a maximal pressor response (3.4 micrograms/kg/min compared to 13.4 micrograms/kg/min, p < 0.05). Magnesium-deficient dogs also had a significantly lower threshold dose for ventricular premature beats (0.8 microgram/kg/min compared to 2.7 micrograms/kg/min, p < 0.05). Acute administration of magnesium sulfate restored pressor sensitivity and ventricular premature beat threshold to normal levels in the magnesium-deficient dogs. Threshold dose for ventricular tachycardia beat was similar in both normal and magnesium-deficient dogs, and threshold was raised significantly in both groups by acute administration of magnesium.