Salivary and serum lithium concentrations were measured simultaneously in 118 manic-depressive patients. Lithium concentration in saliva was 2.24 +/- 0.35 times higher than in serum. An equation to calculate serum lithium concentration from salivary measurements was derived: Li serum = 0.36 Li saliva + 0.13. Psychotropic drugs had no effect on the salivary:serum ratio. Eighteen patients were followed for several weeks. A significant correlation coefficient between salivary and serum lithium concentrations was found in thirteen of the eighteen patients studied.