Abstract1)Exposure to light of dark‐grown gherkin seedlings causes an expansion of the cotyledons and inhibits the elongation of the hypocotyls. As was found earlier for the light‐induced synthesis of phenolic compounds, at least two photoreactions seem to be involved: photoresponse I, the “high energy reaction”, and photoresponse II, the phytochrome reaction.2)The gherkin seedlings contain an enzyme which, purified, oxidizes indoleacetic acid (IAA)1) slowly. The IAA oxidase activity of a crude homogenate depends on phenolic cofactors; derivatives of p‐coumaric acid. Ferulic acid derivatives, flavonols and ascorbic acid, the latter two only from the cotyledons, antagonize this stimulation. The IAA oxidase activity of a homogenate from cotyledons changes on standing due to enzymatic reactions involving the modifiers.3)The photo‐inhibition of hypocotyl elongation can be counteracted by supplying IAA externally, indicating that light lowers the level of IAA. Application of the naturally occurring p‐coumaric acid cofactor of IAA oxidase to hypocotyl sections from dark‐grown gherkin seedlings results in growth inhibition.4)Qualitatively there is agreement between light‐stimulated synthesis of the p‐coumaric acid ester and inhibition of hypocotyl elongation, and between synthesis of IAA oxidase inhibitors and expansion of the cotyledons. The postulated causal relationship between light‐dependent phenol synthesis and photoinhibition of hypocotyl elongation can only exist, however, if but a fraction of the phenolic compounds participates in the IAA regulating system, and the enzymatic oxidation of IAA is coupled to the destruction of phenols.5)In at least one case the IAA oxidase hypothesis seems inadequate to explain the phenomena: the lag for inhibition of hypocotyl elongation in blue light is much shorter than