Performance in cognitive tasks which require the subject to wait and/or to process a large amount of information can be disrupted by an increase in impulsive-like behaviour. Accordingly, a decrease in impulsive-like behaviour can improve performance in such tasks. Conversely, impulsive-like behaviour may improve performance in cognitive tasks where simple and fast responses and/or only little information processing is required. Thus, impulsivity constitutes a confounding factor in studies of cognitive function. Impulsive-like behaviour may be modified by serotonergic (5-HT) activity, with underactivity in 5-HT neurotransmission increasing impulsivity and vice versa. Drug- or lesion-induced alteration in 5-HT neurotransmission may, therefore, constitute suitable tools to investigate the role of impulsivity in animal tests of cognitive function. Benzodiazepines also increase impulsive-like behaviour, possibly by decreasing 5-HT neurotransmission. Hence, the effects of modulation of 5-HT systems and of the benzodiazepine-binding site on performance in animals tests of cognitive function will be discussed. It is predicted that the effects of manipulations of serotonergic activity or of benzodiazepine administration depend upon the nature of the response required, and that these effects may be mediated through changes in impulse control.