The effects of a synthetic growth promoter, 4‐ethoxy‐l‐(p‐tolyl)‐S‐triazine‐2,6 (1H, 3H)‐dione [TA], on growth and gravireaction ofZea maysL. (cv. LG 11) roots were investigated. In horizontal, intact roots, pretreatment with TA at 4 × 10−4Minhibited the gravireaction. If the pretreated roots were rinsed with a buffer solution before incubation, the TA effect was reduced, indicating that a continuous presence of TA was necessary for its maximal activity. On the other hand, the TA pretreatment (1×10−5, 1×10−4and 4 × 10−4M) promoted the elongation of these roots. The TA effect was stronger for illuminated roots than for those kept in darkness. TA also decreased the lateral curvature of half‐decapitated roots maintained vertically in light. This indicates that the action of TA could be associated with some growth inhibiting substances prod