A comparative study has been made of the uptake by and translocation from roots of intact barley plants of six herbicides and a systemic fungicide (four triazines, diuron, 2,4-dichloro-phenoxyacetic acid (2,4-D) and ethirimol). Relationships between uptake and transpiration rate are discussed in the light of the physico-chemical properties of these compounds, notably their partition coefficients in oil/water systems and their dissociation constants. Apart from 2,4-D, sorption of these compounds appears to be a passive process. At pH4 the uptake of 2,4-D seems to be influenced by metabolism; not only may the concentration of this compound in the transpiration stream be considerably greater than that in the medium surrounding the roots but absorption by roots is markedly reduced at low temperatures and by sodium azide. The initial rate of uptake of these compounds correlates reasonably well with their partition coefficients in olive oil/water or n-dodecane/water systems; likewise the concentration in the transpiration stream is greater for lipophilic than for lipophobic substances. Whereas the hydrogen ion and calcium concentrations of the ambient medium appear to have no effect on the uptake of compounds with low pK's, the uptake of those substances which protonate between pH4 and pH6 is affected by them. These findings are discussed from the viewpoint that the pathways of transport of lipophilic and lipophobic compounds across the roots may differ. Although there is some evidence that retention by roots can limit transport to shoots, there is no simple inverse correlation between the total concentration of the different substances in the roots and that in the transpiration stream. This question is discussed in a subsequent paper.