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We investigated the change of root net woton excretion of seedlings of Triticum aestivum L. md Zea mays L. with daily variation of illumination ising a multi-channel pH-stat system. We found an in-:rease of net proton excretion during darkness and a lrop after the beginning of illumination. Inhibition of zarotenoid biosynthesis by norflurazone and photoxidation of chlorophylls did not change the periodicity 3r its induction. The induction of diurnal periodicity was possible with blue, green and red light. After induction the oscillation of net proton excretion continued for at least two cycles under constant light. We conclude that net H+ excretion of wheat and maize roots may be regulated by an endogenous clock or by a signal from the leaves. The nature of such a hypothetical signal remains unknown.