Dimethyl silicones of viscosities 0-25, 10, and 125 St were applied to leaf surfaces and their effects on the rates of transpiration and photosynthesis were measured. The effect of the 10 and 125 St materials lasted more than 16 days, decreasing both photosynthesis and trans piration. The diffusion coefficients of water vapour and carbon dioxide through 10 St silicone were measured in a separate experiment and it is shown that the leaf results can be interpreted in terms of an almost complete silicone cover over the stomata. Calculations show that for materials with /%2q /Dc0 greater than 1 • 6 ( free air ratio ), provided they have no physiological effects, the most effective mode of action would be complete blockage of a fraction of the stomates rather than partial blockage of all. The amount of water saved by doubling the stomatal resistance of three crop types (short grass, farm crops, and trees) in two contrasting environments (Southern England and Cali fornia) is calculated, and it is inferred that anti-transpirants would be most effective when applied to well-watered tall crops in regions of high evaporative demand. Stomata are the channels for the flow of carbon dioxide and water vapour into and out of leaves. The resistance they impose is a greater fraction of the whole for water vapour than it is for carbon dioxide, so an increase in stomatal resistance should affect water-vapour transfer more than carbon dioxide. This will increase the 'production ratio' (PjT) defined as the ratio of rate of photosynthesis to rate of transpiration for unit difference in concentration of both gases. Management of stomatal resistance could have important effects in both agriculture and hydrology. Most experiments with anti-transpirants have used chemicals that interfere with stomatal physiology and make stomata close when sprayed on leaves, e.g. phenyl-mercuric acetate (PMA), dodecenylsuccinic acid (DSA), and nonenyl succinic acid (NSA). However, the effective concentration ranges of these chemicals are very small, and the effects are brief, so they need applying often (Davenport, 1967; Monteith, Szeicz, and Waggoner, 1965). Cetyl alcohol, which will decrease evaporation from an open water surface, has also been used; it does decrease transpiration, but severely damages the plants. Angus and Bielorai (1965), using silicone films on leaves, found transpiration was considerably de creased with little plant damage or check in growth and Malcolm and Stolzy (1968) showed that silicone fluids slightly decreased the transpiration of orange seedlings both in the greenhouse and in the open, though the change was not significant. The experiments reported here investigate the effects of silicone compounds