In the cat, fronto-parietal mu rhythms belonging to the 'high frequency range' (35-45 c/sec) develop as the animal becomes immobile in an attitude of focused attention. Two cortical foci were identified, one anterior in the precruciate area, the other posterior in the periansate cortex (posterior parietal area). A thalamic focus, in an area belonging to the medial portion of the posterior thalamic group, appears to be a pacemaker for the posterior cortical focus alone.