Lithium balance studies were performed in 19 patients suffering from mania and 6 patients suffering from depression. The following results were obtained: (1) The mean daily requirement for lithium in the manic patients was 52 mM, in those with depression 30 mM (additional requirement in manic patients 73%). (2) Renal elimination of lithium, after optimal blood lithium levels had been reached, was 76% in mania and 97% in depression (retention in manic patients 21%). (3) In mania there was an unchanged lithium half-life time (12-13.5 h). (4) In mania and depression no significant differences in lithium and creatinine clearance were noted. (5) Standard diet or unrestricted sodium chloride administration did not significantly influence the lithium requirement or lithium retention. After exclusion of a renal or dietetic cause for increased lithium requirement or retention during mania, the existence of a 'lithium pool' dependent on the presence of a manic psychosis seems probable. As a result of this, somatic influences on endogenous psychosis have to be taken into account.