Pressure gradients in the lacunar system of Egeria densa Planch. were measured by attaching constant-volume manometers to cut surfaces of the shoot system. Upon illumination, the total pressure of the lacunar gas increased up to 30 kPa above the initial pressure, due to the increase in the internal oxygen pressure in the lacunae. The rate of pressurisation was not constant throughout the lacunar system because the more active regions of the shoots pressurised more rapidly than older shoot portions with lower chlorophyll contents. It can be proven mathematically that the resulting pressure gradients (approximately 2 kPa) within shoots during the pressurisation phase could not generate a flow-through ventilation of the type found in some emergent macrophytes, but that a small gas displacement down the stem would occur. The total pressure reached its maximum within 1 h of illumination; at this time, the pressure had equilibrated throughout the shoot lacunae and mass flow was completed. Such pressure gradients are, therefore, of little importance to steady-state aeration, but may relate to the need to rapidly establish a constant partial pressure gradient within a plant with long non-photosynthetic stems.