Genetically modified (GM) tomatoes, carrying thaumatin gene encoding sweet-tasting protein may be a component of diet with high sensory values, constituting a valuable source of nutrients and substances with health-promoting role. Good utilization and lack of effect on animal growth, value of hematological parameters, concentration of immunoglobulins and most of chemical blood parameters of laboratory rats were demonstrated in the nutritional studies on fruits of tomato GM plants. The biological response of the rats, receiving GMO, or its isogenic equivalent in the diet was recognized as similar. However, the unfavourable effect of the diets, containing addition of tomatoes with the recombined thaumatin, on the degree of oxidative degradation of DNA of the rats’ liver was recorded. At the same time, no effect of the discussed diet component on value of the remaining parameters of oxidative state of tissue of the mentioned above organ and its histological image was observed.