Acetolactate synthase (ALS), the first common enzyme in the biosynthetic pathway leading to branched-chain amino acids, is the target site for several classes of herbicides. We have synthesized a pyrimidyl-oxy-benzoate and pyrimidyl-thio-benzenes as ALS inhibitors, and examined their inhibitory activities on pea ALS. $I_{50}$ values of the inhibitors ranged from 0.1 to 8.2 mM. Pyrimidyl-oxy-benzoate showed a mixed-type inhibition with respect to pyruvate, and the inhibition was not dependent on incubation time. Dual inhibition analyses of pyrimidyl-oxy-benzoate versus a feedback inhibitor, leucine, and a sulfonylurea revealed that these inhibitors were competitive with each other in their binding to ALS. This suggests that these two classes of inhibitors, pyrimidyl-oxy-benzoate and sulfonylurea, may bind to the regulatory site of ALS.