In the monkey (baboon or squirrel monkey) displaying a high level of vigilance, a particular type of spontaneous ECoG rhythm develops when the subject is motionless and fixes its attention on a visual target in its surroundings. This activity, whose frequency averages around 18 c/sec, is distributed over two restricted cortical foci, one over the SI hand area (Brodmann's fields 1 and 2) and the other over the posterior parietal cortex (area 5). These rhythms are blocked by the least body movement (but not by eye movements). In view of their location and reactivity, the rolandic portion of these rhythms belongs to the 'mu' type as described in man. The suggestion is made that this synchronized activity favours the maintenance of immobility as well as the fixation of attention on a non-somaesthetic target by temporarily blocking both somaesthetic (in area SI) and somatopraxic integration (in area 5).