The present study evaluated the reactivity of cortical rhythms in 15 Alzheimer's disease (AD) patients, 7 Lewy body dementia (LBD) patients and 9 control subjects using a 165 SQUID whole-head MEG system. The absolute power values of the rhythms recorded over different areas over the brain (frontal, parietal, temporal, occipital) were analysed in the 3-47Hz frequency range. The cortical reactivity of the alpha (9-14Hz) and pre-alpha rhythms (7-9Hz) during open and closed eyes conditions differentiated the control group from the patient groups and moderate AD from severe AD and LBD groups, respectively. The cortical reactivity of the slow-band (3-7Hz) obtained by comparing a simple mental task and the rest discriminated the severe AD group from the other groups. In addition, spectral coherence analysis in the alpha band showed that the loss of coherence in AD and LBD patients mainly involved long connections. These results suggest that investigations on rhythms reactivity and spectral coherence might help on the study of the dementias with different etiology.