Acetohydroxyacid synthase (AHAS) i s the site of adion of herbicides of different chemical classes, such as imidazolinones, sulfonylureas, and triazolopyrimidines. lnhibition of AHAS causes the accumulation of 2-ketobutyrate (2-KB) and 2-aminobutyrate (2AB) (the transamination produd of 2-KB), and it has been proposed that the phytotoxicity of these inhibitors i s dueto this accumulation. Experiments were done to determine the relationship between accumulation of 2-KB and 2-AB and the phytotoxicity of imazaquin to maize (Zea mays). lmazaquin concentrations that inhibit growth of maize plants also cause the accumulation of 2-KB and 2-AB in the shoots. Supplementation of imazaquin-treated plants with isoleucine reduced the pools of 2-KB and 2-AB in the plant but did not proted plants from the growth inhibitory effeds of imazaquin. Conversely, feeding 2-AB to maize plants increased 2-KB and 2AB pools to much higher levels than those observed in imazaquintreated plants, yet such high pools of 2-KB and 2-AB in the plant had no significant effect on growth. These results conclusively demonstrate that growth inhibition following imazaquin treatment i s not due to accumulation of 2-KB andlor 2-A6 in plants. Changes in the amino acid profiles after treatment with imazaquin suggest that starvation for the branched-chain amino acids may be the primary cause of growth retardation of maize.