Anabolic androgenic steroids (AAS) are known as doping agents within sports and body-building, but are currently also abused by other groups in society in order to promote increased courage and aggression. We previously showed that 14 days of daily intramuscular injections of the AAS nandrolone decanoate (15 mg/kg) reduced the extracellular levels of the dopaminergic metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the nucleus accumbens shell using microdialysis. The aim of the present study was to investigate whether the same dose regimen of nandrolone decanoate may affect the activities of the dopamine-metabolizing enzymes monoamine oxidases A and B (MAO-A and MAO-B). A radiometric assay was used to determine the activities of MAO-A and MAO-B in rat brain tissues after 14 days of daily i.m. nandrolone decanoate injections at the doses 3 and 15 mg/kg. Gene transcript contents of MAO-A, MAO-B and cathecol-O-methyltransferase (COMT) were measured with quantitative real-time reverse transcription PCR. 3 mg/kg of nandrolone decanoate significantly reduced the activity of both MAO-A and -B in the caudate putamen. 15 mg/kg of nandrolone decanoate significantly reduced the activity of MAO-A in the amygdala and increased the gene transcript level of MAO-B in the substantia nigra. In conclusion, imbalanced MAO activities may contribute to explain the impulsive and aggressive behaviour often described in AAS abusers. The reduced MAO activities observed are in line with our previously presented findings of decreased extracellular levels of DOPAC and HVA in the rat brain, indicating decreased monoaminergic activity following repeated AAS administration.