We examined the effects of brassinolide (BL) and/or an auxin (indole-3-acetic acid) on ethylene production and elongation in the primary roots of maize (Zea mays). When these two hormones were applied exogenously, both increased ethylene production. Before the tenth hour after treatment began, the influence of lAA was more evident than that of BL; the reverse was found beyond 10 h. When these hormones were treated simultaneously, the increase in level of ethylene was greater than the sum of effects by each hormone. Such a positive interaction was also recorded for changes in the activity of ACC synthase and the expression of its gene. For ACC oxidase, however, the two hormones had no apparent influence. When applied separately, neither affected root elongation nor proton extrusion. However, when given in combination, both phenomena occurred. Our results suggest that BL interacts with lAA to promote ethylene biosynthesis and elongation in roots. Therefore, it is possible that brassinolide acts by inducing auxin, which then stimulates both ethylene production (at the early stage) and root development.