1. Nine severe rapid cycling manic-depressive patients were treated with a magnesium preparation, Magnesiocard 40 mEq/day in an open label study for a period up to 32 weeks. 2. Magnesiocard was found to have clinical results at least equivalent to those of lithium in about 50% of these patients. These results were obtained in an exploratory study and should be interpreted with caution. 3. The possibility that Magnesiocard could replace or improve the efficacy of lithium as a preventive treatment of manic-depressive illness merits further clinical investigation.