Within the last ten years it has been established by GC-MS that indole-3-butyric acid (IBA) is an endogenous compound in a variety of plant species. When applied exogenously, IBA has a variety of different effects on plant growth and development, but the compound is still mainly used for the induction of adventitious roots. Using molecular techniques, several genes have been isolated that are induced during adventitious root formation by IBA. The biosynthesis of IBA in maize (Zea mays L.) involves IAA as the direct precursor. Microsomal membranes from maize are able to convert IAA to IBA using ATP and acetyl-CoA as cofactors. The enzyme catalyzing this reaction was characterized from maize seedlings and partially purified. The in vitro biosynthesis of IBA seems to be regulated by several external and internal factors: i) Microsomal membranes from light-grown maize seedlings directly synthesize IBA, whereas microsomal membranes from dark-grown maize plants release an as yet unknown reaction product, which is converted to IBA in a second step. ii) Drought and osmotic stress increase the biosynthesis of IBA maybe via the increase of endogenous ABA, because application of ABA also results in elevated levels of IBA. iii) IBA synthesis is specifically increased by herbicides of the sethoxydim group. iv) IBA and IBA synthesizing activity are enhanced during the colonization of maize roots with the mycorrhizal fungus Glomus intraradices. The role of IBA for certain developmental processes in plants is discussed and some arguments presented that IBA is per se an auxin and does not act via the conversion to IAA.