Water loss or transpiration is an important physiological process that affects the main quality characteristics of fresh mushrooms, such as saleable weight, appearance and texture. A loss in weight of only 5% may cause fresh produce to lose freshness and appear wilted and it is an important parameter to be considered while designing packaging. A setup was developed to monitor the mass loss of mushrooms under various surrounding temperatures (4, 10 and 16 degrees C) and relative humidity (76%, 86% and 96%). Mass loss of whole button mushrooms (Agaricous bisporous) was measured and transpiration rates were calculated at all the experimental conditions tested. Transpiration rate ranged from 0.14 mg/(cm(2) h) to 2.5 mg/(cm(2) h) over all the combinations of temperature and humidity tested. Humidity was the variable with the greatest effect on transpiration rate. A mathematical model was developed to understand the evolution of water loss with temperature and relative humidity. The fitting was very good and the distribution of residuals was normal. The model was validated at 13 degrees C and good agreement was found between experimental and predicted data. (C) 2007 Elsevier Ltd. All rights reserved.