Blocking or desynchronization of the central beta rhythm prior to and parallel to voluntary movement was found in 31 of 33 normal subjects investigated. The beta blocking was short lasting (1-3 sec), very often accompanied by blocking of the mu rhythm and localized in the central region. Phasic beta desynchronization was also observed after somatosensory stimulation. The beta rhythm showed a bilateral symmetrical blocking pattern with unilateral movement or stimulation. Patients with unilateral cerebral ischaemia showed an asymmetric blocking response and therefore demonstrated a high degree of hemispheric independence of the rhythmic generating systems. Central beta desynchronization is, therefore, a normal physiological phenomenon caused by activation processes of the sensorimotor cortex and detectable on the scalp very easily with closely spaced electrodes.