The aim of this study was to confirm the presumption that the duodenal flow of soluble nitrogen fractions: soluble non-ammonia nitrogen (SNAN), soluble protein and long-chain peptides nitrogen (SPLPN), soluble short-chain peptides and free amino acids nitrogen (SSPFAAN) and free amino acids (FAA) will be increased after the application of ruminally protected tablets containing soya protein and amino acids (Met, Lys, His). Control animals received the same mixture but in ruminally non-protected (powder) form. The experiment was carried out on three lactating Holstein cows of average weight of 523 kg fitted with ruminal and duodenal cannulas. The experiment was divided into 4 periods of 14 d (10 d preliminary period and a 4-d experimental period). In the first period one cow received the tablets (T) and the other two received the powder (C) with the same composition. In the subsequent period the rate of animals was antipodal. Cows were fed on diet based on maize silage, lucerne hay and a supplemental mixture. Powder or tablets consisted of purified soya-protein HP 300, Lys, Met and His. The experimental treatment did not significantly (P>0.05) influence duodenal flow of SNAN, SPLPN and SSPFAAN. In case of FAA significant (P<0.05) increase of flow (in g/day) was obtained in T group for Met (1.25 vs 0.35), Lys (2.07 vs 0.76), His (2.13 vs 0.49) and Arg (0.37 vs 0.15) against the control C.