Transpiration of wild oat (Avena fatua L.) plants was markedly reduced after foliar treatment with barban (4-chloro-2-butynyl-m-chlorocarbanilate), asulam (methyl sulfanylcarbamate), dichlorfop methyl (4-(2′,4′-dichlorophenoxy)-phenoxypropionic acid methyl ester), difenzoquat (1,2-dimethyl-3,5-diphenyl-1 H-pyrazolium) or benzoylprop ethyl (ethyl-N-benzoyl-N(3,4-dichlorophenyl)-2-aminopropionate). Suppression of transpiration increased with increasing herbicide rates. Difenzoquat and dichlorfop methyl at 1.12 kg/ha reduced transpiration by more than 50% within 2 days after spraying. Barban, asulam and benzoylprop ethyl did not reduce transpiration to this level until about 12 days after spraying. When wild oats and barley (Hordeum vulgare L.) or wheat (Triticum aestivum L.) were grown together, removal of the weed with these herbicides resulted in significantly heavier barley and wheat plants with more tillers per plant than in the untreated control. The earlier removal of wild oat competition with dichlorfop methyl and difenzoquat treatments resulted in the production of more dry weight and culms per plant of barley and wheat than with the slower-acting barban and benzoylprop ethyl.