A mixture of free amino acids was incubated in vitro with rumen liquor or administered as either a single dose or continuous infusion into the rumen of sheep fitted with a simple rumen cannula and reentrant duodenal cannula. The rate of disappearance of amino acids from the incubated liquor was slow and varied for individual amino acids. The most rapidly disappearing amino acid was cysteine. Glycine, alanine, histidine and valine were decomposed more slowly reaching a level of degradation not exceeding 70% of their initial concentration after 24 h of incubation. Phenylalanine was not degraded during 24 h incubation. In vivo, amino acids rapidly disappeared from the rumen at varied rates and only a small proportion of the amino acids introduced into the rumen reached the duodenum upon continuous infusion (3.4%) or single injection (7.4%). Most resistant to decomposition were thyrosine, cysteine and phenylalanine, while proline, methionine, asparagine, serine and glutamine were most readily decomposed.