There is considerable evidence supporting a role of the polyamine system in the etiology and pathology of mental disorders. Changes in the expression and activity of polyamine anabolic/catabolic enzymes, as well as in the levels of individual polyamines, have been found in many psychiatric conditions, including schizophrenia, mood disorders, anxiety, and suicidal behavior. Recent microarray studies have found that spermidine/spermine-N¹-acetyltransferase (SAT1, SSAT), the key enzyme in charge of the polyamine catabolic pathway, is downregulated in brain tissue of individuals who were depressed and died by suicide. To provide further insight into the downstream effects of altered SAT1 expression, we developed a quantitative gas chromatography-mass spectrometry method for measurement of polyamine concentrations in postmortem human brain tissues. This protocol employs a conventional electron ionization method with total ion and selected ion monitoring. This method can accurately measure the levels of the polyamines putrescine, spermidine, and spermine from very small quantities (1-50 mg) of postmortem brain tissues, with quantitation limits down to 10 ng/g of wet tissue for putrescine and 100 ng/g for spermidine and spermine.