A light-induced auxin-inhibitory substance, indolylacetonitrile (IAN), was isolated from light-grown cabbage (Brassica olearea L.) shoots. The IAN content in cabbage shoots increased in response to light. Cabbage hypocotyl elongation is inhibited by 10SM of IAN in 90 min but promoted after 48 hr. Light-induced growth inhibition is caused by the degradation of 3-indolylmethylglucosnolathe (IMG) to IAN which is, in turn, converted to the growth hormone indole-3-acetic acid (IAA).