Abstract Height and fresh weight of sorghum ( Sorghum bicolor L. var GA 522 DR) grown in sand were reduced by metolachlor (2-chloro- N-(2-ethyl-6-methylphenyl)- N-(2-methoxy-1-methylethyl) acetamide) (0, 0.125, 0.25, 0.5, or 1 ppmw) applied preemergence. Significantly different responses were obtained from plants grown at two light intensities (270 and 27 μein/m 2/sec). When grown in nutrient solution containing 0, 0.0156, 0.0625, 0.25, 1, 16, or 64 ppmw metolachlor, shoot and root lengths were inhibited by metolachlor; fresh and dry weights of shoot, root, and total plant decreased as metolachlor concentration increased. Carotene content (micrograms per gram fresh weight) in sorghum leaves was decreased by metolachlor. Specific activity of carotene synthesized from [2- 14C] mevalonic acid by carrot ( Daucus carota L.) disks was reduced 50% by 10 −5 and 10 −4 M metolachlor. Thus, terpenoid biosynthesis is influenced by metolachlor. Gibberellins are terminal products of plant terpenoid biosynthesis, and GA 3 reverses metolachlor inhibition of growth at specific ratios of GA 3 and metolachlor but not at other concentrations. Thus, one effect of metolachlor on plants may be an inhibition of GA synthesis that results in shoot and root growth reductions as metolachlor concentration increases. Other growth responses of plants to metolachlor are unexplained.