Ethylene stimulated the elongation of maize mesocotyls of whole seedings under red light. Maximum elongation was obtained with 1μl1^-1 ethylene. The length of mesocotyls was still much longer than in ethylene-free air when ethylene concentrations were increased to 10, 100 or 1, 000μl1^-1. Ethylene also induced lateral expansion of mesocotyls at ethylene concentrations of 0.1μl1^-1 or above and the diameter increased at higher ethylene concentratins. Carbon dioxide, in the range of 1-4%, also stimulated mesocotyl extension and expansion. Maximum growth of mesocotyls was obtained in a mixture of ethylene and carbon dioxide. Removal of either endogenously evolved ethylene or carbon dioxide or of both gases reduced elongation. In contrast to the effects in red light, ethylene inhibited the growth of mesocotyls in darkness while carbon dioxide inhibited this effect of ethylene. Thus, ethylene and carbon dioxide acted cooperatively under red light and antagonistically in darkness.