Inhibitors of protein and RNA synthesis (cycloheximide, puromycin, chloramphenicol, and actinomycin D), as well as Co(++), induce opening of the hypocotyl hook of bean seedlings during the early stage of the opening period both in the darkness and red light. The response is transitory, however, complete straightening of a hook can not be achieved in the presence of these agents. These agents abolish the response of hooks to red illumination. They also block the suppression of hook opening caused by IAA and ethylene. The response and sensitivity to GA are not affected by the inhibitors. Inhibitors of DNA synthesis (FUDR and mitomycin C) have no effect on hook opening. It appears that in this growth response RNA and protein synthesis are more immediately involved in ethylene action than they are in the cell elongation process or the action of GA thereon.The results indicate that phytochrome does not induce hook opening simply by activating genes whose products directly promote growth. It is suggested that the regulation of ethylene formation by light and auxins may be exerted by way of influences on tissue levels of phenolic inhibitors of ethylene biosynthesis.