Electrical potential differences between electrodes inserted in eight selected sites of the trunk of a Turkey oak (Quercus cerris) and the ground have been recorded continuously since November 1997. The mean amplitudes of daily electric variations were determined for time intervals when the meteorological and atmospheric electricity conditions were undisturbed. It was found that the mean daily amplitudes show a characteristic annual fluctuation. Both in 1998 and in 1999 two peaks were observed: the first maximum appears suddenly about the end of March, and the second one was seen in late June, July. Between November and March the mean amplitudes were smaller by about one order of magnitude.