SUMMARY1. Two cases have been reported in which normal symmetrical alpha waves were observed in the presence of a large and thick unilateral subdural hematoma.2. These cases necessitated experimental investigation on short‐circuiting effect of hematoma on cortical electrical potentials, which had been insisted upon by various authors and could be accepted from the standpoint of common sense. In this connection modifying effects of the structures overlying the brain on cortical potentials were also examined.3. Modification of cortical electrical potentials by the skull and the scalp consists of spatial diffusion, reduction in amplitude and changes in frequency spectrum. Spatial diffusion was found very extensive and the most responsible structure for causing diffusion and reduction in amplitude was found to be the skull. Relative attenuation of fast waves by those structures was observed, but the relationship between the degree of attenuation and wave frequency was not simple. The cause was discussed.4. Observation was made on modifying effects of a rubber sheet and a cotton layer soaked in normal saline, each of which was inserted between the cortex and the skull, on cortical potentials as well as on potentials applied to the surface of the cortex or the dura, and following inference has been made. Short‐circuiting effect on cortical potentials may be manifested when the hematoma is relatively fresh and not so small nor so thin. Normal symmetrical EEG may be observed in the presence of a large unilateral subdural hematoma when it developes very chronically and its capsule much thickly and when main sources of alpha waves participating in the scalp EEG on the hematoma lie outside the cortex covered by the hemat