Various forms of wounding result in increases in hydrostatic pressure in the laticiferous phloem tissue of Hevea brasiliensis; regular tapping, a form of controlled wounding, probably causes a similar response. When a tree is ring-barked, there is a transient fall in turgor immediately above and below the ring, presumably owing to loss of latex during cutting. This is followed by a pressure increase which is particularly marked above the ring, suggesting the accumulation of metabolites. Isolation of an ‘island’ of tissue, by cuts down to the wood, results in a steep fall in turgor within it, although a relatively slow recovery may follow. Partially isolated panels show smaller falls and a more rapid rise. It appears that a functional phloem connexion permits a more rapid recovery of osmotic and turgor pressures following latex losses, both in the isolation experiments and in normal tapping. Growth regulators such as 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) increase latex yields by prolonging latex flow after tapping. In untapped trees treatment with 2,4,5-T results in a slow and fairly small increase in turgor pressure, but this effect may not be apparent if trees are regularly tapped. When the tapping cut is opened, there is a rapid fall in pressure immediately under the cut. This loss in turgor spreads through the latex-vessel system as latex flows towards the cut, but recovery is apparent near the cut even before flow ceases. Pressure gradients indicate a rapid formation of a localized resistance to flow at or near the cut surface. This process appears to play a major part in restricting flow. The increased yields which result both from regular tapping and after 2,4,5-T treatment appear to result from a delay in this sealing process. The mechanisms by which the barrier to flow is built up and delayed by 2,4,5-T treatment are not clearly understood.