Despite the fact that waking and sleeping EEG activities are well known in cercopithecinae, a striking random activity was noted in 3 macaque species: M. mulatta, M. fascicularis, M. nemestrina. It occurred in bursts of monophasic slow waves (250-400 msec) at 3/sec (2.5-4/sec, according to the moment and the individual at 50-480 microV amplitude, with a waxing and waning aspect. The duration was 2-24 sec. It predominated along the midline, in the precentral and supplementary motor areas; hence its name, the delta vertex rhythm. It occurred either isolated or several times within the same REM or waking episode. In some monkeys it was more or less frequent on different recording nights; however, in other monkeys it could be totally absent. Its mean total duration, for a 12 h recording, was 0.45% for M. mulatta and 0.065% for M. fascicularis. No significant correlations were found between its occurrence and the age, sex, the moment of occurrence of eye movements and sawtooth waves.