A convenient system via a perfect liquid junction was developed for measuring the trans-root electric potential (TRP) of a tree in the field. This system enabled continuous measurement of the TRP of an old kaki tree (Diospyros kaki, Japanese persimmon), which showed clear diurnal oscillation throughout two successive years. The level of the TRP on the average for every ten days had two minima (ca. 70 mV) in mid winter and mid summer, and two maxima (ca. 30 mV) in October and May. A regular seasonal change in the amplitude of the oscillation was observed; it was at its maximum (ca. 70 mV) in early spring just before the sprouting of new leaves, and at its minimum (ca. 10 mV) during summer and early autumn when leaves were fully expanded. The relationship between the TRP, water transport and rainfall is discussed.