Albumin, β₂-microglobulin and free light chains were determined in urine in nine manic-depressive patients before and at intervals during three months of lithium treatment (longitudinal study). The same determinations were carried out in twenty-seven manic-depressive patients who had been treated with lithium for 3 months to 20 years and also in a control group (transversal study). There were no statistically significant changes in urinary excretions of albumin, β₂-microglobulin and free light chains during the longitudinal study. In one patient albumin excretion gradually increased during the study and remained elevated on reexamination 1 year later. No significant differences were found between the lithium treated patients and control subjects in the transversal study in either albumin, β₂-microglobulin or free light chain excretion. It is not clear whether the increased and sustained albumin excretion in one of the patients was due to lithium or was coincidental. The study shows that in most patients lithium treatment does not affect renal protein excretion.