Abstract Tomato and lettuce plants were exposed to vapour of the herbicide 14C 2,4-D butyl at concentrations in the range of 1–660 pg/l for 6, 24 or 72 hr. The relationship between herbicide uptake, measured as amount of radiolabel in the plant, and vapour concentration was linear and independent of the duration of exposure for both species. The rate of uptake of herbicide vapour at any one concentration by tomato was about twice that of lettuce. Immediately after exposure, the leaves of lettuce plants contained similar amounts of herbicide residue, whereas leaves at the base of tomato plants usually contained the least herbicide, and those at the apex the most. In terms of leaf area, both species had highest residue contents in the apical leaves. Different exposure periods did not affect the amounts of herbicide residue in the apical leaves or the leaf below, indicating that herbicide present in the apex is largely due to uptake and not to translocation. Both species developed visible symptoms of phytotoxicity following exposure, the severity being directly related to the amount of herbicide vapour received. Leaf contents of herbicide residue in tomato plants 40 days after exposure were related to the total plant content measured immediately after exposure; the leaf below the apex at the time of exposure retained about 17% of the herbicide originally present. Shoot dry weight, 40 days after exposure, was reduced by 10% following uptake of about 250 ng/plant in tomato, whereas for lettuce, about twice this amount of 2,4-D butyl was required to cause noticeable weight reduction. Doses of this magnitude are of the order of 10 −5 the amount used in field application, and so it is clear that 2,4-D vapour is a potential hazard to tomato and lettuce crops.