Abstract Amino acid absorption as measured by proto-aortic differences was studied in rats given 10% protein diets ad libitum. Free plasma amino acid (PAA) levels were determined in portal and aortic blood sampled at 2200 h (low intestinal absorption time), 0400 h were beef, casein, rapeseed, soybean and gluten. These proteins were also submitted to an in vitro digestion process by which free amino acids and low molecular weight peptides (digestion products) were collected as they were released after pepsin and pancreat in hydrolysis. Even though most essential amino acids (EAA) were preferentially hydrolysed during the 6-h digestion, the pattern of release was different for each protein. Essential PAA of intestinal effluent (porto-aortic difference) were compared with both in vitro digestion products and protein EAA. In most instances, correspondence between EAA patterns and digestion products was higher. Correlations were rather low for beef and casein (r=0.70), but higher with rapeseed and particularly with soybean and gluten (r=0.92 between PAA of 2200 h intestinal effluent and peptide fraction of in vitro digestion products for soybean, and r=0.93 between 0700 h intestinal effluent and peptide fraction for gluten). It can thus be stated that compared to protein amino acid composition, in vitro digestion products appear to be better predictors of amino acid bioavailability.