The ability to detect membrane damage in plant leaves by the pressure chamber technique was evaluated. Membrane damage was induced by freezing and thawing, absorption of the host-specific toxin victorin, ozonation, inoculation with Helminthosporium maydis, and spraying with the phytotoxin, fusicoccin. The pressure-volume relationships, i.e. the volume of sap expressed from a leaf with incremental increases in pressure, were compared in leaves with intact or damaged membranes. Where membrane damage was widespread throughout the leaf, sap was expressed at pressures as low as one tenth of those needed in leaves with intact membranes and at low pressures the amount of water expressed from leaves with damaged membranes was up to 10 times that from leaves with intact membranes. Further, the pressure-volume curves of healthy leaves became linear when the leaf turgor potential was reduced to zero, but were non-linear in leaves with damaged membranes. Ozone treatment, inoculation with H. maydis, or spraying with fusicoccin damaged only a proportion of the leaf cells, and pressure-volume relationships more nearly resembled those obtained with healthy leaves rather than those obtained with freezing and thawing and absorption of victorin.It is concluded that the pressure chamber can be used to observe membrane integrity, but its ability to detect damage may be limited to conditions in which the majority of cells in the leaf are damaged.