Four Hereford Yen Friesian mature steers equipped with rumen, duodenal and ileal cannulas were fed on either grass silage alone (S) or silage supplemented with fishmeal (150 gkg-1 silage dry matter intake; SFM) to assess effects on rumen fermentation, nutrient flow to and disappearance from the small intestine. The silage was a primary growth of perennial ryegrass with a total-N content of 24.5 gkg-1 dry matter (DM). Ruminal concentrations of acetate, butyrate and ammonia-N were not different between treatments and averaged 51.7, 8.9 and 8.4 mmoll-1, respectively. However, molar proportion of propionate was higher on SFM compared to S, 18.7 and 17.6 (SEM 0.18, P < 0.05), respectively. The amount and efficiency of microbial protein synthesis were not different between treatments and averaged 71.0 gd-1 and 35.8 g Nkg-1 organic matter digested in the rumen, respectively. Fishmeal supplementation increased the flow of total-N ( P < 0.05), non-ammonia-N ( P < 0.05), and total amino acids ( P < 0.05) at the duodenum. Amino acid disappearance was increased by fishmeal supplementation, 488 and 717 gd-1 (SEM 28.6, P < 0.05) for S and SFM, respectively. The apparent absorption coefficient for the total amino acids from the small intestine [(expressed as duodenal - ileal)/duodenal] was also increased, 0.57 and 0.72 (SEM 0.028, P < 0.06) for S and SFM, respectively.