A technique, using leaf disks, has been developed to study the penetration of isotopically labelled compounds into leaves under conditions where there is no appreciable change in the concentration of the external solution and no subsequent translocation. In this preliminary survey, the leaves of Phaseolus vulgaris and Coleus Blumei were employed to investigate the entry of 2,4-dichlorophenoxyacetic acid (2,4-D), labelled in the carboxyl group with 14C. Over 3 days there is no loss of 14C to the atmosphere from treated leaves of Phaseolus. The rate of penetration is enhanced when (a) the leaves are young, (b) the water status is lowered, (c) the temperature is raised (Q10=2.3–2.8), and (d) a surface-active agent is added to the external solution. Penetration is also favoured by a decrease in the pH, the relation indicating that both ions and molecules enter. Penetration is greater in the light and prior illumination of the tissues positively affects the subsequent rate in the light, but not in the dark. In both the light and the dark considerably more 2,4-D penetrates the abaxial surface of Phaseolus leaves. For Coleus an even greater difference between surfaces is found in the light but not in the dark. For both species in the light the rates of entry into both surfaces are proportional to their respective stomatal densities. The simultaneous addition of indoleacetic acid to the external solution caused more 2,4-D to enter Phaseolus leaves, but the addition of triiodobenzoic acid restricts entry. The rate of penetration remains constant over 24 hours and between 0.1 and 200 mg./l. the rate is linearly related to concentration. Subsequent to entry, the 2,4-D is in a form which does not diffuse from the tissue into buffer or exchange with unlabelled 2,4-D. Moreover, no outward movement takes place from treated tissue which has been frozen and thawed. These findings are discussed in relation to previous work on foliar penetration. It is concluded that at least with Phaseolus penetration largely takes place through the guard cells and/or accessory cells.