• Downbeat nystagmus developed in a 67-year-old hypomagnesemic woman while she was receiving lithium carbonate for depression. This nystagmus abated each time lithium carbonate therapy was withdrawn, and no alternative causes of nystagmus were demonstrated. However, this nystagmus occurred despite serum lithium carbonate levels in the nontoxic range. Total-body magnesium deficiency may have enhanced the toxic effect of lithium carbonate on cerebellomedullary connections.