A pressure transducer was used to measure internal gas pressures (pI) in the lacumar system of Egeria densa Planch. Under illumination, pI values up to 25 kPa above atmospheric developed in shoots in unstirred water. Upon circulation of the water, pI values decreased to 50–90% of the maximal values in unstirred water. Positive pressures, indicating partitioning of photosynthetic O2 into the lacunae, were recorded in both re-circulating and open-circuit flow. When the shoot was darkened, negative internal pressures (5–10 kPa below atmospheric) developed.The pressure gradient (Δp) required to drive O2 transport in the lacunar system was calculated from the Hagen-Poiseuille equation. This gradient was 9.15 × 10−4 kPa m−1. Manometric measurements of pI and pressure transfer rates showed that transient gradients of approximately 0.9 kPa m−1 developed following dark to light changes. However, pressure equilibration (mean rate = 0.02 m s−1) resulted in rapid attainment of constant pI under steady-state conditions. It is proposed that O2 transport in Egeria is sustained by simple diffusion, assisted by advective movements under internal pressure gradients that develop following changes in external conditions.