AbstractThe structure of the imidazolinone herbicides consists of three distinct moieties: the imidazolinone ring, the carboxylic acid and the backbone. The effect of changes in each of these on herbicidal activity, crop selectivity and AHAS enzyme inhibition has been studied. Though both whole‐plant and enzyme activity were drastically affected by changes in the carboxylic acid or imidazolinone ring portions of the molecule, a variety of backbones and of substituents on the backbones afforded good activity. Methyl‐isopropyl was found to be the best combination of substituents on the imidazolinone ring. While pyridine backbones generally gave the most active herbicides, benzene backbones led to the strongest enzyme inhibition. A QSAR study in the pyridine series generated two equations which proved useful for guiding the analog program toward the synthesis of potent heteropyridyl compounds. Selectivity in wheat is dependent upon differences in the rate of metabolism of key groups. Rapid metabolism of either the imidazolinone ring or backbone alkyl groups occurs rapidly in soybeans compared with susceptible weeds.