A four-finger tapping sensor and associated software have been developed for the investigation of timing and rhythm performance and mechanisms in normal subjects and their disruption in neurological disorders. The tapping sensor comprises four electronic touch pads and pacing lights. A personal computer (PC) is used to control visual and auditory pacing, record the time and pad of each tap, and carry out several performance analyses including graphing, phase-space plots, calculation of spectra and autocorrelations, filtering and descriptive statistical analysis. A study was conducted to investigate disruptions of timing and rhythm in subjects with Parkinson's disease (PD). Seven Parkinsonian and ten control subjects undertook paced and unpaced finger tapping tests. The hastening phenomenon-tapping asynchronously at a speed faster than the pacing-was seen with similar occurrence in both normal and PD subjects and appears to be due to perceptual difficulties. No evidence was seen of an increased variability of tapping at particular frequencies, contrary to previous reports. Festinated tapping, in which subjects cycled between acceleration to near-maximum speed and abrupt slowing down, was seen only in PD subjects. As noneof these subjects showed significant hastening, it appears that hastening and festination are unrelated phenomena. Inspection of variations of finger tapping intervals gave no evidence for the presence of deterministic chaos in the control of rhythmic tapping. In speed tests performed with and without a weight attached to the finger, it was found that, for reasons which remain unclear, normal subjects increased their maximum tapping speed with the weight attached.