Acetohydroxyacid synthase (AHAS) is the first common enzyme in the biosynthetic pathways for valine, leucine, and isoleucine. AHAS is the target site for four classes of structually unrelated herbicides, the sulfonylurea, imidazolinone, triazolopyrimidin, and pyrimidyl-oxy-benzoate chemical families. AHAS has been partially purified from dark-grown pea shoots approximately 2,000 fold. We have synthesized new sulfonylurea and imidazolinone herbicides, and examined their biological activities using the pea enzyme. $I_{50}$ values for inhibition of AHAS by sulfonylureas and imidazolinones tested ranged from 3 to 50 nM and from 67 to about 500 nM, respectively. Sulfonylurea chlorosulfuron and imidazolinone Cadre were shown to be competitive and mixed-type inhibitor with respect to pyruvate, respectively. The inhibitions of AHAS by both sulfonylurea and imidazolinone were time-dependent and biphasic.