The usefulness of amino acid digestibility measurements as indices of availability can be evaluated by comparing the amino acid composition of distal ileal digesta (where absorption is believed to be complete) with that in the rectum. Two semipurified diets containing either soybean or rapeseed oil meals were each fed to four castrated male pigs of 60 kg liveweight. The pigs were sacrified and samples of digesta were removed from the stomach, duodenum, jejunum, ileum, cecum, colon, and rectum. The concentrations of nitrogen and the proportions of 14 amino acids in the samples were studied. Digesta taken from the duodenum contained the greatest concentration of nitrogen; the nitrogen concentration decreased through the small intestine, and then remained constant through the large intestine. Comparing the ileal and rectal digesta, there were no significant differences in the proportions of valine, arginine, serine, tyrosine, threonine, phenylalanine, and aspartic acid for both diets. This was also true for lysine for the soybean meal diet and for methionine and alanine for the rapeseed meal diet, so that for these amino acids, digestibility should be a good estimate of availability. However, there were lower proportions of leucine plus glycine, proline, and glutamic acid in the rectal than in the ileal contents for both diets, and for lysine in the rapeseed meal diet, suggesting that digestibility values would overestimate availability. In contrast, there were higher proportions of isoleu-cine in the rectal than in the ileal contents for both diets, and of methionine and alanine for the soybean meal diet, suggesting that digestibilities would underestimate the availabilities of these amino acids.Partition of the digesta and the water-soluble digestion marker (polyethylene glycol) in the stomach precluded reliable quantitation of digesta flows through the ileum, but the experiment suggests that such quantitative measures would provide a basis for more valid estimates of amino acid availabilities than measurements of the amounts of amino acids voided in the feces.