The predominant mechanism of peptide breakdown by rumen micro-organisms is aminopeptidase. Thus acetylation of the N-terminus of peptides inhibits their degradation by rumen micro-organisms in short-term incubations with rumen fluid in vitro. An experiment was undertaken to determine if adaptation of the rumen microbial population would take place when acetylated peptides were fed for a prolonged period, which would enable the microbial population to break down the protected peptides and thus decrease their nutritive value. Three adult sheep, fitted with permanent rumen cannulae, received a maintenance hay/concentrate diet to which was added, at each meal, 20 g of casein enzymic hydrolysate ('peptides') or 20 g of peptides previously treated with acetic anhydride. The diets were fed for 28 d in a 3 x 3 latin square and samples were taken during the last 7 d. Fermentation products and NH3 concentrations indicated that acetylated peptides remained less degradable than untreated peptides. There was a trend towards increased proteolytic activity with acetylated peptides, and dipeptidase activity increased by 18% and 28%, respectively, compared with untreated peptides and control treatments. Activity against N-acetyl-Ala also increased when acetylated peptides were fed, but it remained only 13% of the rate of Ala hydrolysis. No increase was found in the rate of ammonia production from acetylated peptides in animals receiving acetylated peptides-this rate was 26% of that found with untreated peptides-and acetylated peptides continued to persist for longer in the rumen than untreated peptides after feeding. Thus it was concluded that the rumen microbial population did not adapt to utilize acetylated peptides.