Two right-handed patients with infarction involving the forebrain commissural fibres produced irregular but rapid repetitive movements with their left hands while producing normal movements with their right hands as well as having normal oral expression. The abnormality was more conspicuous when producing slow tapping patterns. When required to reproduce rapid tapping patterns of around 5 beats/sec, one of the patients produced a comparatively regular tapping pattern. Thus, we believe that both hemispheres are equipped with the faculty to produce rapid repetitive patterns and that only the left hemisphere is responsible for the temporal processes involved in producing required repetitive patterns.