To investigate central processing time in patients with progressive supranuclear palsy and Parkinson's disease, reaction times were measured using tasks with different levels of cognitive complexity but with the same motor response. In patients with Parkinson's disease, the additional central processing time required for more complex situations was no different from that in control subjects, suggesting that cognitive aspects of the reaction time procedures tested were possibly too simple to reveal a slowing of thought processes in these patients. Conversely, the central processing time was increased in patients with progressive supranuclear palsy compared with both Parkinson's disease and control subjects. The increase was associated with impairment in frontal lobe test performance. These results confirm that a slowing of central processing is a prominent feature of the cognitive disturbances of progressive supranuclear palsy and, furthermore, suggest that this slowing may be related to striatofrontal dysfunction.