A tapping test in which the tasks required a synchronous response to a periodic sound signal by the finger tapping was performed by 27 normal subjects and 146 patients with Parkinson's disease. Some patients could not maintain a synchronous response at a critical frequency of 2.5 or 5 Hz (taps per second) and showed a hastened tapping of 5 ∼ 6 Hz, independent of the input signal frequencies, i.e., the ‘hastening phenomenon.’ The other patients and normal subjects could respond synchronously up to 5 ∼ 7 Hz. Autocorrelational analysis of the sequential tapping intervals suggested that the synchronized response was performed by a feedback control with the input signal as target except at 2.5 and 5 Hz, whereas the hastened tapping of patients was a random process. The frequency-dependence of the error in the tapping response had two maxima around 2.5 and 5 Hz. The hastened tapping was interpreted as representing an intrinsic oscillation in the central nervous system, which would be masked in normal subjects but released in patients with Parkinson's disease. Around 2.5 or 5 Hz the error of response becomes so large that some patients can no longer maintain the synchronized response and show a hastened tapping due to this intrinsic oscillation.