Two patients with neurological signs of magnesium depletion complained of oscillopsia and manifested downbeat nystagmus. Serum magnesium levels measured less than 1.0 mg/dL. Hypomagnesemia resulted from debilitating intestinal disease in both patients and apparently was aggravated in one case by failure to add magnesium sulfate to the patient's regimen for total parenteral nutrition. Neither patient had any of the neurological conditions reported with downbeat nystagmus. Thus, downbeat nystagmus in the primary position occurs with the metabolic derangement of severe magnesium depletion. Another apparent metabolic cause of downbeat nystagmus is thiamine deficiency. Downbeat nystagmus also may occur from a partial deficiency of the metabolic cofactors, magnesium and thiamine.