A method was developed to measure lithium concentrations in the human brain using in vivo 7Li- and 1H-magnetic resonance spectroscopy (MRS). Lithium concentrations measured by MRS in 10 lithium-treated bipolar patients were at the half level of those measured in serum. Serial measurements indicated that lithium concentrations in the brain increased markedly during manic episodes, while serum concentrations were unchanged. These findings suggest that in vivo measurements of lithium concentrations in the brain, combined with measurements of concentrations in serum, may be useful in monitoring the effects of therapeutic drugs.