The composition of the fruits of non-transgenic or transgenic (GM) cucumbers with genes coding the synthesis of a sweet protein, thaumatin, were compared and effects of feeding the fruits in balanced diets to rats were determined. The transgenic cucumbers contained more protein (20.3 vs 17.9% DM) and less fibre (9.4 vs 11.4% DM) and also had lower Na, K, Ca and Mg contents and higher levels of Fe and Cu in ash than normal cucumbers. Feeding male rats of initial body weight 150 g for 5 weeks on isoprotein diets containing 0 or 15% lyophilized transgenic or non-transgenic cucumbers did not affect weight gain, apparent health status, or relative organ weights of animals. Protein digestibility was slightly but significantly lower (89.2 vs 90.0%), that of crude fibre was higher (28.2 vs 15.0%) in diets containing transgenic than non-transgenic cucumbers, while digestibility of fat and N-free extractives did not differ.