We have characterized the magnetic 10- and 20-Hz rhythms recorded with a whole-scalp neuromagnetometer during different conditions. Sources of the posterior 10-Hz (alpha) rhythm clustered mainly around the parieto-occipital sulcus and, to a lesser extent, around the calcarine sulci, with several generators. Temporal Spectral Evolution (TSE) analysis, used to follow event-related changes in the different frequency bands, showed strong dampening of the alpha within 200 ms after the appearance of a visual stimulus and also during visual imagery. Suppression was often followed by a rebound above the baseline level. The rolandic mu rhythm consisted of 10- and 20-Hz components with different reactivity and source locations. The 10-Hz component seems to be mainly somatosensory in origin whereas the 20-Hz signal also receives contributions from the motor cortex, and even shows 'motorotopy' in its reactivity: the source locations depend in a somatotopical manner on the site of the moving body part. The frequency composition of the posterior spontaneous activity was disturbed in patients with small infarcts of the medial thalamus. It is shown with simulations that a surprisingly small number of synchronized cortical neurons could generate the major part of the recorded oscillatory signal. Finally, some clarifications are suggested to the terminology of brain rhythms.