AbstractThe substituted 1,2,4‐triazolo[1,5‐a]pyrimidines are a new class of highly active herbicides. Protection of Arabidopsis thaliana seedlings from triazolopyrimidine‐induced injury by the branched‐chain amino acids was observed. Acetolactate synthase (EC 4.1.3.18) was isolated and found to be quite sensitive to inhibition. I50values for inhibition of the enzyme from a number of plant sources show little variation and no correlation to whole‐plant response, suggesting uptake, translocation and metabolism play key roles in modulating herbicidal activity. Further studies indicate that these chemicals are slow, tight‐binding inhibitors that are readily dissociated by gel filtration. Some correlations between in‐vitro activity and in‐vivo activity were observed for ortho‐substituted analogs on selected