Ethephon (I) is used commercially to prolong the flow of latex from the rubber tree after tapping (Yield stimulation). The compound is applied to the bark in the region of the tapping cut and the effect on latex flow is due to the ethylene released by chemical decomposition, since gaseous ethylene itself is also a very effective stimulant. When 14C-I is applied to the bark of a young Hevea seedling, it is absorbed into the plant by processes which appear to be largely non-metabolic. Ethylene formation commences immediately at the site of application, and the gas is quickly translocated throughout the plant. Translocation of I to all parts of the plant also occurs and the accumulation of 14C in the bark above the zone of application is greater than that below. Chromatographic analysis has shown that compounds other than I remain in the plant tissue. Experiments using 14C-I have shown that detached leaves are able to convert a considerable proportion of the compound to at least twelve non-volatile acid products. One of these is a conjugate of I with an unidentified material. A major component of the products is 2-hydroxyethylphosphonic acid (II), which is itself converted to a number of compounds in leaves. The application of I to bark from mature Hevea, results in the formation of a single substance which is also a conjugate of I. Neither I nor II is effective in inducing the formation of ethylene from endogenous precursors in vegetative Hevea tissue. Ethylene is poorly metabolized by Hevea leaves and the evidence available indicates that it is unlikely that any of the compounds produced from I are metabolites of ethylene.