In red-light grown corn (Zea mays L. cv Brio42.HT) coleoptiles, cortical microtubules adjacent to the outer cell wall of the outer epidermis reorient from transverse to longitudinal in response to auxin depletion and after phototropic stimulation in the lighted side of the coleoptile. This was used as an in situ assay of cellular auxin concentration. The fluence-response relation for the blue light-induced reorientation is compared with that for first positive phototropism and the dose-response relationship for the auxin-dependent reorientation. The result supports the theory by Cholodny and Went, claiming that phototropic stimulation results in auxin displacement across the coleoptile. In terms of microtubule orientation, this displacement becomes even more pronounced after preirradiation with a weak blue light pulse from above.