It has recently been shown that indole-3-butyric acid (IBA) is formed in maize, via acetylation of indoleacetic acid, by a microsomal membrane fraction, ATP and acetyl-CoA. A similarity of this reaction to the carboxylation of acetyl-CoA is proposed and therefore the effects of herbicides specific for the inhibition of acetyl-CoA carboxylase were tested on IBA synthesis. Aryloxyphenoxypropionates and cyclohexanediones inhibited the growth of the maize seedlings as predicted, whereas IBA biosynthesis was promoted after the application of cyclohexanediones. Herbicides with different targets, however, did not increase IBA synthesis, IBA synthetase was not influenced in sethoxydim-tolerant maize seedlings as compared to susceptible plants. It was shown that IBA synthetase from dicotyledonons plants was not affected by sethoxydim application, whereas IBA synthesis in wheat was modulated in the same may as in maize.