Flavin-containing monooxygenase (FMO), previously reported both from hepatic and extrahepatic tissues, including brain, catalyze the oxidation of certain xenobiotics and drugs that contain a nucleophilic heteroatom. Psychoactive drugs, including the antidepressant imipramine, are substrates for the brain FMO. Since FMO-mediated metabolism of these drugs might contribute to local pharmacodynamic modulation within the human brain, the metabolism of imipramine by human brain FMO was studied in further detail. In the present study, the FMO activity was determined in human brain microsomes by estimating the actual amount of imipramine N-oxide formed. It was then compared with the corresponding activity measured using substrate (imipramine)-stimulated rates of nicotinamide adenine dinucleotide phosphate (NADPH) oxidation, which was significantly higher than the activity estimated as the amount of N-oxide assayed using high-pressure liquid chromatography (HPLC). The brain FMO activity was measurable only in the presence of detergents (sodium cholate or Lubrol PX) or in microsomes that were freeze-thawed several times. The activity was inhibited by an antibody to rabbit pulmonary FMO, but an antiserum to the rat liver NADPH cytochrome P-450 reductase had no effect indicating that cytochrome P-450 was not involved in the above metabolic pathway. The optimum pH for N-oxidation of imipramine was found to be 8.5; thermolability experiments indicated that the FMO activity was completely lost only after the incubation of brain microsomes at 45 degrees C for 20 minutes. An FMO purified to apparent homogeneity from a human brain had a molecular weight of 71,000 Da. The purified enzyme cross-reacted with the antibody to rabbit pulmonary FMO and efficiently catalyzed the metabolism of imipramine to its N-oxide. The human brain clearly contains an active FMO system, and it is conceivable that such enzymes are significantly involved in the local metabolism and modulation of pharmacological and/or toxic effects of certain xenobiotics, including psychoactive drugs.