We analyzed spontaneous MEG activity, measured with a whole-head neuromagnetometer in 7 healthy, relaxed adults. Rhythmic activity concentrated over the rolandic and parieto-occipital regions and contained spectral components in the 10 and 20 Hz ranges, each showing distinct reactivity. Sources of the rhythmic activity in different frequency ranges were localized within the brain and related to the sites of the auditory, visual, and somatosensory projection cortices. The sources of the rolandic mu rhythms concentrated in restricted areas of 3-5 cm in diameter, close to the hand projection area in the first somatomotor cortex. The posterior alpha activity received a significant contribution from the vicinity of the parieto-occipital fissure. Combination of spectral analysis, reactivity of the rhythms with respect to tasks or stimuli, and source localization will allow focusing on well-specified cortical rhythms and assist in gaining an understanding of the functional significance of cortical rhythms.