Four ruminally cannulated Finnish-Ayrshire dairy cows were used to study effects of type of grass silage (primary vs secondary) and level of concentrate supplementation on concentration and the flow of soluble non-ammonia N (SNAN) in the liquid phase of digesta entering the omasum. The treatments in a 4 x 4 Latin square design consisted of either primary growth of grass silage (PG) or secondary growth of grass silage (SG) each with either 8.1 or 12.1 kg/d of concentrate (air dry basis). Digesta entering the omasum was collected, and SNAN fractions (free amino acids, peptides and soluble protein) in the digesta were assessed using ninhydrin. The microbial contribution to SNAN was estimated using ¹⁵N as a microbial marker. Concentrations of free amino acids, peptides and soluble protein N averaged 22.3, 59.8 and 23.1 mg N/l, respectively. PG diets tended (P=0.09) to increase peptide N concentration in omasal digesta compared with SG diets, whereas level of barley concentrate did not affect concentrations of SNAN fractions. Peptide N constituted the largest proportion of SNAN in omasal digesta, supporting the previous observation that hydrolysis of peptides to AA is the most limiting step in rumen proteolysis. The microbial contribution to SNAN averaged 0.71, indicating that a substantial proportion of the SNAN was of microbial origin. Soluble dietary NAN flow averaged 9.1 g N/d, accounting for approximately 0.05 to 0.08 of total dietary NAN flow.