Transitory changes in fresh weight of leaf lettuce. Which might be associated with the changes in ambient conditions, were analysed, thereby a suitable method for monitoring fresh weight was investigated. The fresh weight is temporarily lost by (1) illumination, (2) temperature rise at a constant relative humidity, (3) increase in vapor pressure deficit (VPD), and (4) increase in wind velocity, while it is gained under the opposite conditions. The fresh weight transitorily changed within 30 min after the state of exposure to different conditions. When the light conditions changed from dark to light, both of the transpiration rate and water saturation deficit of the leaves increased. The increase in temperature at a constant relative humidity caused loss of fresh weight, but the weight remained stable under a constant VPD. It was concluded that the transitory changes in fresh weight of leaf lettuce could be ascribed to the changes in transpiration rate. Therefore, the fresh weight as an indicator of plant growth has to be measured under the conditions that keep the transpiration rate constant.