Four steers, average body weight 260±15 kg, fitted with portal catheters were used in a 4×4 Latin Square design to evaluate the influence of dietary protein degradability in the rumen on peptide and amino acid fluxes across the gastrointestinal tract. Dietary protein degradability was regulated by using different protein sources and the diets were calculated to contain 130 g CP/kg and 9·62 MJ ME per kg DM. Plasma concentrations of amino acids were analysed before and after acid hydrolysis of samples first subjected to chemical deproteinization and physical ultrafiltration, and peptide amino acids (PAA) were calculated as the difference between total and free amino acids (FAA). Portal blood flow and arterial concentrations of FAA and PAA were not affected by protein degradability or by diet. Venoarterial concentration difference and net portal flux of FAA tended to increase (P < 0·10) with increase of degradable protein intake. Portal-arterial concentration difference (P < 0·05) and net portal flux (P < 0·10) of PAA increased linearly as dietary protein degradability increased. The proportion of PAA in total amino acid (FAA+PAA) net flux was not modified by dietary protein degradability or by diet, and the mean value as a proportion was 0·32. The major PAA absorbed were glutamate, leucine, aspartate and lysine for all diets, accounting in total for 0·50 of PAA flux. The results demonstrate that PAA may contribute significantly to AA flux across the portal-drained viscera (PDV) of steers, and both FAA and PAA net fluxes can be affected by degradable protein intake.