Cyclic changes in the net photosynthetic rate and the transpiration rate were observed in peanut leaves. Both rates changed synchronously with a period of approximately 50 min. The variations in the cyclic changes of the photosynthetic rate and the transpiration rate were large (0-30 mgCO2·dm-2·h-1 and 0.1-3.0 gH2O·dm-2·h-1 respectively). Cyclic change in the transpiration rate in leaves was synchronized over the whole plant. These results suggest that cyclic change in stomatal aperture, often called stomatal oscillation, is responsible for the cyclic changes in the photosynthetic and transpiration rates. Change in intercellular CO2 concentration due to the change in photosynthetic activity of mesophyll cells is not a probable cause of the stomatal oscillation because oscillatory transpiration was sustained in low CO2 air. When the transpiration of the leaves outside the leaf chamber was suppressed, the stomatal oscillation of the remaining leaves was diminished. This implies there is a relationship between the water status of the leaves and the occurrence of stomatal oscillation. Simultaneous measurement of whole-plant transpiration and water absorption revealed the oscillation of water absorption rate occurred which 10-20 min behind the oscillation of the transpiration rate. Oscillation of the water balance of the plant, calculated from the difference between rates of water absorption and transpiration, was also observed. This seemed to be caused by the time lag in water absorption. These results suggest that the stomatal oscillation of peanut leaves is caused by the whole plant oscillation of water balance due to the low capacity of water supply relative to transpiration.