We investigated the role dopamine and noradrenaline in the modulation of attention in Parkinson's disease (PD) patients. We observed that PD patients with mild and moderate motor disability did not differ in their attentional accuracy in easy tests, but the severe PD group was slightly disrupted in a more arduous test of attention. Attentional accuracy was not affected by withdrawal of dopaminergic drugs in mild or severe PD patients. The movements of severe PD patients were slower, and withdrawal of dopaminergic drugs aggravated motor slowing more in severe PD patients. Clonidine (0.5 and 2 microg/kg) retarded accuracy of performance in the most difficult attention test in mild PD patients, but had no effect in the severe PD group. Clonidine had no effect on movement times. These data suggest that a defect in noradrenaline release may contribute to the impaired accuracy of attention in severe PD patients and that dopamine may be important for maintaining rapid motor responding.