1. The mangrove Avicennia germinans, maintained under semi-steady-state glasshouse conditions, exhibited stomatal oscillations. 2. Continuous, simultaneous measurements of transpiration, photosynthesis, leaf temperature and water potential indicated that cycling was synchronized throughout the plant and was rapidly propagated by changes in water potential. 3. The frequency of cycling ranged from 30 to 48 min, usually with large amplitudes, and resulted in considerable fluctuations in transpiration and photosynthesis. 4. Cyclic variation in water potential lagged behind those in conductance and transpiration by 2-7 min. 5. Oscillations were induced by increases in vapour pressure deficit (VPD) and osmotic potential of the substrate and eliminated by decreases in VPD, substrate osmotic potential, root temperature and root resistance. 6. Oscillatory behaviour appeared to be induced by any factor that brought about rapid changes in hydraulic flow through the plant. 7. These results are discussed in relation to feedback mechanisms that regulate stomatal movement.