Studies of reaction time in Parkinson's disease (PD) have suggested a selective deficit in simple reaction time (SRT), compared with choice reaction time (CRT). This finding has been interpreted as a deficit in motor preprogramming but could involve other factors, such as attentional focussing and stimulus predictability. Moreover, not all studies show the same selective deficit, possibly because of differences in patient selection and treatment effects. The neurochemical basis of RT deficits in PD remains unclear. Accordingly, the contribution of cognitive factors to impaired RT was assessed in a large group of PD patients, including early untreated cases, and performance was examined in relation to clinical variables and the effect of treatment in longitudinal study. Motor output was constant in both SRT and CRT tasks. In the SRT task, all stimuli required a response; in the CRT task, subjects were required to respond to only one of the two possible stimuli