The experiment was performed to determine whether transgenic maize containing the Bt gene (MON 810) and soyabean meal produced from glyphosate-tolerant plants (Roundup Ready, MON 40-3-2) compared with nontransgenic plants can affect the performance parameters, milk composition, blood serum metabolite profiles and transfer of tDNA into milk of cows. The experiment was carried out from the third week before parturition to the 305^th day of lactation on 40 Polish Holstein-Friesian cows, which were allotted to 4 groups of 10 animals according to body weight, milk yield and parity. The cows in all groups were fed a total mixed ration (TMR) formulated according to IZ-INRA (2009). The concentrate mixtures added to TMR contained: nonmodified (traditional) maize and soyabean meal (group TM_G/TS); traditional maize and genetically modified (GM) soyabean meal (group TMG/MS); GM maize and traditional soyabean meal (group MM_G/TS); or GM maize and GM soyabean meal (group MM_G/MS). There were no significant differences between transgenic and non-transgenic feeds in productivity, milk composition and blood metabolite profiles such as: β-hydroxybutyric acid (BHBA), free fatty acids (FFA), glucose, insulin and progesterone. The transgenic DNA sequences of MON 810 and RR soyabean meal were not detectable by PCR in milk.