Incorporation of the Bacillus thuringiensis (Bt) toxin gene or the glufosinate (phosphinothricin) tolerance (Pat) gene into the genetic construction of maize and sugar-beets did not significantly affect chemical composition and digestibility of crude nutrients and has no consequences on energy content, as demonstrated. This is supported by results from feeding trials carried out with poultry, pigs and ruminants. Although the rations were formulated to have high incorporation rates of transgenic or alternatively isogenic feedstuffs, their production power did not differ. From the nutritional point of view the results confirm the thesis that these types of genetic modifications (first generation of GM-plants without substantial changes in composition) do not significantly affect chemical composition and energetic feeding value. Fragments of plant DNA could be detected in some animal tissues (e.g. muscle from chicken), but fragments of transgenic DNA were not found.