Abstract Effects of chronic l-deprenyl administration on hyperactive behaviour and brain monoamine levels were studied in spontaneously hypertensive rats (SHR) and Wistar–Kyoto (WKY) rats. SHR were hyperactive, impulsive and had impaired sustained attention when tested with a multiple 2-min fixed interval (FI) 5-min extinction (EXT) schedule of reinforcement. Even low, 0.25 mg/kg, doses of chronically-administered l-deprenyl reduced the impulsiveness (bursts of responses with short interresponse times) of SHR, without altering the general hyperactivity or the impaired sustained attention. The drug had no effect on WKY behaviour. The levels of noradrenaline (NA), dopamine (DA), serotonin (5-hydroxytryptamine, 5-HT) and their metabolites, measured in neostriatum, nucleus accumbens and frontal cortex, showed that l-deprenyl effectively inhibited monoamine oxidase (MAO) activity. These results suggest that impulsiveness is a behavioural component that may be operating independent of the other components, like hyperactivity and deficient sustained attention, and that can be reduced by chronic MAO-B inhibition with l-deprenyl in this strain of rats. The positive effect of l-deprenyl on impulsiveness is discussed as due either to normalization of an asymmetric dopaminergic activity in the nucleus accumbens, or to a restoration of normal DA function in the prefrontal cortex.