Data on 22 subjects treated with fluoxetine suggest that magnetic resonance spectroscopy (MRS) of fluorine-19 can measure brain concentrations of fluoxetine/norfluoxetine in vivo. Fluoxetine accumulates in the human brain relative to plasma, with brain concentrations of fluoxetine/norfluoxetine ranging up to 10.7 micrograms/ml. Brain concentrations may reach a plateau between 6 and 8 months of treatment. The apparent concentration in brain relative to plasma is 20:1, roughly parallel to brain antidepressant concentration ratios in animal studies.