By radioautography of whole plants it has been possible to show that different herbicide molecules display different mobilities as they move from treated leaves to stems and roots. 2,4-Dr IAA, ATA and MH* form a mobility series increasing in the order given. In this series there is evidence for transfer of the tracers from phloem to xylem increasing in the same order. Tracers so transferred move in the transpiration stream to all transpiring leaves and so may enter leaves that are exporting assimilates. Dalapon is readily mobile in plants but it has little tendency to accumulate in roots. Sugar derived from assimilation of CO2 resulting from hydrolysis of urea applied to leaves also moves freely; it does not transfer from phloem to xylem. Movement of 2,4-D IAA, ATA and MH in potato tuber tissue is in the same order as in barley plants; the latter movement therefore is related to penetration of the leaf tissues. Monuron apparently moves only in the apoplast of leaf and potato tuber tissues.