To assess the spatial extent and temporal behavior of rolandic rhythms we recorded neuromagnetic signals from four healthy subjects with a 24-channel magnetometer. The subjects performed self-paced thumb movements or the motions were triggered by electrical stimulation of the median nerve at the wrist. The main frequency components of the magnetic mu rhythm signals centered at 10 and 20 Hz. Both components were completely suppressed during the movement and increased substantially 0.5-2.5 s after it; the 20-Hz component reacted about 300 ms faster. The rebound was stronger after self-paced than after stimulated motion, and after contra- than after ipsilateral movement. The reactive source areas were identified for both frequency ranges, and they clustered on partly overlapping cortical areas of 6-8 cm2 wide along the course of the central sulcus. The 10-Hz rhythmic oscillations occurred predominantly at the primary somatosensory hand cortex; the sources of the 20-Hz signals were slightly more anterior. We hypothesize that the 10-Hz signal is a true somatosensory rhythm whereas the 20-Hz activity is essentially somatomotor in origin.