Patients with idiopathic Parkinson's disease and control subjects were given two types of time estimation task. In the verbal estimation task subjects were required to judge the duration of time intervals of 10, 30 and 60 seconds, signalled by the examiner. Operative estimation required subjects to indicate when intervals of 10, 30 and 60 seconds had passed. Parkinsonian patients off L-Dopa medication were less accurate than control subjects in both tasks. They underestimated the time intervals determined by the examiner in the verbal estimation task, while in the operative estimation task parkinsonian subjects overestimated the required time intervals compared with controls. The administration of L-Dopa to the parkinsonian patients led to time judgements that were similar to those of control subjects. The present findings lend support to the hypothesis of the role of dopamine in the modulation of the internal clock.