A series of experiments examined the effects of 6-hydroxydopamine (6-OHDA)-induced depletion of forebrain norepinephrine (NE) on the performance of a visual detection (spatial localization) task. The behavioral paradigm used was an analogue of Leonard's 5-choice serial reaction time task for humans. The 6-OHDA lesion of the dorsal noradrenergic bundle (DNAB) produced a 98% depletion of the NE content in the neocortex, and a much smaller depletion (32%) of the NE content in the hypothalamus. As reported previously, performance of visual discrimination was unaffected by DNAB lesions, even when the discrimination was made more difficult by decreasing the intensity of the visual stimuli. However, the lesion produced a significant decrease in accuracy and a significant increase in omissions when a burst of loud white noise was presented just prior to the onset of the visual discriminanda. Similarly, a significant decrease in discriminative accuracy was produced in the rats with forebrain NE depletion by systemic administration of the psychomotor stimulant, d-amphetamine (0.2 to 0.8 mg/kg). In both of these experiments, the lesion-induced discrimination impairment was not magnified by reducing the brightness of the visual discriminanda, suggesting that the behavioral impairment was not caused by a decreased ability to detect the visual stimuli. In addition, the lesion impaired discriminative accuracy when the visual discriminanda were presented at an unpredictable rate. The implications of these behavioral impairments produced by forebrain NE depletion for theories of catecholamine involvement in attentional processes and arousal are discussed in terms of a possible role for the DNAB in controlled or "effortful" processing.