Six experiments examined the effects of 6-hydroxydopamine (6-OHDA) induced lesions of the nucleus accumbens septi (NAS) on performance of a spatial discrimination. The behavioral paradigm used was an analogue of Leonard's 5-choice serial reaction time task for humans. The 6-OHDA lesion produced an 87% depletion of dopamine (DA) in the NAS, only a minor (25%) depletion of DA in the anterior caudate, and a 75% depletion of neocortical noradrenaline (NA). The lesion transiently attenuated both the speed and impulsivity of responding on the baseline schedule, but did not affect discriminative accuracy. In addition, the lesion attenuated the increase in premature responding caused by both systemic administration of D-amphetamine and bursts of loud white noise, presented just prior to the onset of the visual discriminanda. However, the lesion did not affect discriminative accuracy in these 2 conditions. The lesion also only had extremely minor effects on performance of this paradigm when the intertrial intervals were unpredictable. These results contrast with the previously reported pattern of behavioral effects resulting from forebrain NA depletion in the same behavioral paradigm. They therefore complete a double dissociation of effects on accuracy and vigour of responding, supporting theories of a division of arousal-type processes.