Plasma and erythrocyte magnesium concentrations were measured in 79 clinic outpatients who had histories of bipolar or unipolar primary affective disorder and who were being chronically treated with lithium carbonate (60 patients) or placebo (19 patients). Although slight differences in the mean concentration of magnesium in plasma and erythrocytes were noted with lithium treatment, diagnosis, and sex, the differences overall failed to achieve statistical significance. In contrast to prior reports demonstrating increases in plasma magnesium during acute lithium carbonate treatment of affectively ill patients, these data suggest that the pre-lithium steady state is achieved during chronic lithium treatment.