The effects of feeding rats transgenic potatoes with overexpression of 14-3-3 protein (line J₂) and repression of ADP ribosylation factor (line J₁) and control potatoes (line Desiree) were compared. Potatoes of line J ₂ contained much more crude protein (16.2% in DM) in comparison with control potatoes (10.7%) and transgenic line J₁. Modification of 14-3-3 protein and ADP ribosylation also affected mineral composition, especially the Fe content. Feeding rats for 5 weeks with isoproteinous diets containing 20% dried transgenic or unmodified potatoes had no significant effect on the growth of the animals, did not negatively affect nutrient digestibility - the digestibility of fat and fibre in the diets containing tubers of transgenic lines J₁ and J₂, were higher, and a significantly higher digestibility of the organic matter of diets with J ₂ potatoes was found. The relative weight of selected internal organs, morphological parameters and most blood biochemical parameters did not differ significantly among the groups of rats fed diets containing dried transgenic and unmodified potatoes.