Fluoxetine is one of the most commonly used medications to treat mental disorders, however, 30-50% of patients fail to respond to the treatment. Thus, direct noninvasive measurements of Fluoxetine concentration in the brain may give insights into individual variations in their clinical responses. This study aimed to quantify fluoxetine concentrations in the human brain using 19F MRS at 3T. 19F MRS measurements were performed on 20 subjects with stable doses of fluoxetine over a month. The fluoxetine concentration was 30±28.2 μM and significantly correlated with their daily doses; r=0.78, p<0.0001, which promises the utility of 19F MRS for personalized medicine.