A 1-Hz rhythmic event-related potential was recorded at the scalp during performance of a 0.5-Hz tracking task. At cortical motor areas, negative peaks occurred 10-20 ms after peak tracking speeds. Analysis of single sweeps suggested that EEG phase was reset at initiation of the tracking motion and then maintained a constant relationship to wrist speed until task completion. Frequency analysis indicated that rhythm appearance in the averaged potential was predominantly due to phase-locking, because there was no tracking-related increase in 1 Hz amplitude within individual sweeps. While tracking, phase-locking was present over bilateral parieto-occipital and frontal regions, with a slight predominance at the contralateral frontal region. When subjects observed the target motion, phase-locking was localized to parieto-occipital regions. We suggest mental processes such as visual processing, visuomotor coordination and real-time motor planning are reflected in the pacing of localized cortical potential fluctuations.