This paper is concerned with the timing of regular repetitive movements. The two-process model of Wing and Kristofferson attributes variability in self-paced interresponse intervals to imprecision in a timekeeper and to temporal noise in the execution of motor responses triggered by the timekeeper. Assuming independence of timekeeper intervals and motor delays, the variance of each may be estimated from interresponse-interval statistics. Comparison of changes in timing performance associated with alterations in motor-system functioning offer the possibility of a new approach to investigation of this model. Illustrative data are presented from a case study of a patient with Parkinson's disease whose lesions affecting the dopaminergic pathways of the basal ganglia have given rise to asymmetric symptoms, including differences in timing performance of the two hands. Analysis of interresponse-interval variability according to the two-process model indicates that the elevated variability of the side more greatly affected by parkinsonism is attributable to the timekeeper intervals rather than the motor delays.