The role of glutathione transferases (GSTs) in the selectivity of the herbicides alachlor, atrazine, fluorodifen and metolachlor, which are detoxified by glutathione conjugation in plants, was determined in seedlings of maize (Zea mays L.) and the associated weed species Abutilon theophrasti Medic., Digitaria sanguinalis (L.) Scop. Echinochloa crus-galli (L.) Beauv., Panicurn miliaceum (L.), Setariafaberi Herrm. and Sorghum bicolor (L.) Moench. The availability of glutathione was also determined in all species and tissue concentrations were found to be in the range 120-160 PM for all species except D. sanguinalis and S . bicolor, which contained half this amount. GST activities toward the herbicides were determined in crude protein extracts from the plants using HPLC to quantify the biosynthesis of the herbicide conjugates. The specific activities of the GSTs toward the substrates were in the order alachlor > fluorodifen > atrazine > metolachlor in all species except A. theophrasti, where fluorodifen was a better substrate than alachlor. In most cases there was a good correlation between GST activities and the selectivity of the herbicides applied preemergence. In the case of atrazine, GST activities were also related to the relative rates of herbicide conjugation in uiuo. In contrast, there was no simple relationship between glutathione availability and the selectivity of the herbicides. However, with alachlor there was evidence that glutathione availability was limiting GST activity and influencing tolerance.