Avena stem segments, which respond with high amplitude, specificity, and sensitivity to gibberellic acid, were used to study the inhibition of gibberellin-induced elongation by the growth retardant alpha-cyclopropyl-alpha-(4-methoxyphenyl)-5-pyrimidine methanol (EL-531). It was found that EL-531 strongly inhibits gibberellic acid-induced elongation in this system at a concentration of 1 mm. From a double-reciprocal plot of elongation and gibberellic acid concentration, it seems that EL-531 and gibberellic acid do not compete reversibly for the same site of action. Also, because EL-531 effectively inhibits elongation in internodal tissue dissected away from the node and leaf sheath, it cannot be acting primarily by inhibiting the synthesis or transport of the leaf sheath factor(s). Because EL-531 causes lateral expansion of the stem segments as well as increased diameters of epidermal cells, in a manner very similar to the effects of colchicine, it is suggested that EL-531 inhibits gibberellic acid-induced elongation by somehow interfering with the orientation of the products of cell wall synthesis.