In vivo phosphorus-31 magnetic resonance spectroscopy (31P-MRS) was used to measure phosphorus metabolism in the brains of manic patients treated with lithium carbonate. The phosphomonoester (PME) peak was elevated in the manic state in patients with DSM-IIIR bipolar disorder compared to the euthymic state and to drug-free normal controls. Elevation of the PME peak may be caused by lithium-induced accumulation of inositol 1-phosphate in the brain.