The present study sought to determine the roles of the two hemispheres in arousal and the selective components of attention. Ten patients with left and right parietal lesions and ten with left and right temporal lesions participated in the experiment. The hypothesis that posterior parietal lesions, whether left or right, cause two selective attentional deficits, namely, a reduced reactivity to stimuli in the visual field contralateral to the lesion, and a reduced reactivity to any stimulus which occupies a relative contralateral spatial position, was tested by asking the patient to tilt their head either to the left or to the right by 90 degrees and to respond to two stimuli displayed above and on either side of fixation mark. The arousal component of attention was studied by analysing the overall RT to visual stimuli independent of their spatial positions. The results showed that (1) patients with either left or right parietal damage are impaired in shifting attention from the ipsilateral to the contralateral visual field, and, within each visual field, in a direction contraversive to the lesion and (2) these two attentional deficits are more severe after right than after left parietal damage. Furthermore, the results show that the difficulty in maintaining a high level of alertness is a specific deficit of patients with right hemispheric lesion and not of patients with an extinction syndrome, insofar as there is no significant difference in overall RT between patients with parietal and temporal lesions.