The interrelationships between light intensity and the effects of sodium 2:4-dichlorophenoxyacetate on the growth of Helianthus annuus in the early vegetative stage have been studied by subjecting treated and control plants to a range of light intensities from, 1.0 to 0.12 daylight. The growth regulator in varying amounts was applied either as droplets of aqueous solution to the first or second pairs of leaves or as an overall spray. When the amount of the compound is such as to cause small but significant reductions in the relative growth rate, the leaf-area ratio and the ratio of leaf area to leaf weight are likewise depressed but the net assimilation rate is relatively unaffected. Between 0.25 daylight and full daylight the proportionate changes in the relative growth rate, leaf-area ratio, and net assimilation rate induced by sub-lethal amounts of the growth regulator are not greatly modified by the level of light. If the intensity is reduced further to 0.12 daylight, then the reactions of the shaded plants differ markedly from those of unshaded plants. For example, when the plants are shaded both before and after a spray application, the concentration required to cause a 50 per cent. mortality is one-tenth of that demanded for plants receiving full daylight. For less phytotoxic amounts the percentage reductions, relative to the controls, induced in the growth rate of the shoot are more dependent on the intensity after than before the application of either a spray or measured droplets. These greater depressions in shoot growth at 0.12 daylight after the application are linked with comparable depressions in the rate of growth of the leaves of which the first pair are more sensitive. Similarly, the growth rate of the first internode is also depressed more under shade conditions, but that of the second internode at low doses may be greatly increased. For both the leaves and internodes shading before as against shading after the application may have different significant effects on the changes in growth caused by the growth regulator. Under some conditions the interactions between the light intensity before and after the application and the quantity of the growth regulator are exceedingly complex. By using sodium 2:4-dichloro-5-iodophenoxyacetate labelled with radioactive I131, it has also been established that the rate of penetration into leaves is accelerated by a high light intensity after the application, but that the intensity received prior to the application causes no significant effect. Evidence was also obtained that transport to the shoot from the treated leaf is more dependent on the light intensity after the application—the transport being greater in full daylight. It is concluded that a number of factors must be involved in bringing about these differential effects and their relative importance is discussed.