Psychopharmacology (2007) 193:85–96 ongoing behavior, i.e., the inability to stop a behavioral response that has just been initiated. Our observations support the notion of different and separable forms of inhibitory control, and moreover, suggest a differential role for the endocannabinoid system therein. Likewise, lesion studies have suggested that different brain regions are involved in inhibitory control measured in either the 5- CSRTT or stop-signal paradigm, as for instance, lesions of the nucleus accumbens or subregions of the medial prefrontal cortex have been shown to impair inhibitory control in the 5- CSRTT (Christakou et al. 2004; Chudasama et al. 2003; Muir et al. 1996) and not in the stop-signal paradigm (Eagle and Robbins 2003b). In summary, the present study provides evidence for a differential involvement of the endocannabinoid system in independent measures of impulsivity, as SR141716A primarily affected inhibitory control, and neither impulsive choice nor response inhibition, whereas WIN55,212-2 only slightly affected response inhibition. In this regard, the present data add to existing clinical and preclinical evidence demonstrating that distinct measures of impulsivity can be dissociated at a pharmacological and neuroanatomical level (e.g., Chudasama et al. 2003; De Wit et al. 2002; McDonald et al. 2003; Winstanley et al. 2004). Our findings may be of particular interest with respect to the heterogeneity observed in attention-deficit/hyperactivity disorder (Sonuga-Barke 2002) and suggest that possible novel pharmacotherapies targeted at the cannabinoid system may benefit the subtype resulting from poor inhibitory control, but not the motiva- tional style, or delay aversion, subtype. 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