Hindgut fermentation differs from rumen fermentation by a lower methane production and the presence of reductive acetogenesis. Bile acids which are lost into the lower digestive tract may have a promoting effect on reductive acetogenesis in the hindgut. In this experiment it was investigated if bile acids induce reductive acetogenesis in rumen fermentation in vitro. Rumen contents from a fistulated cow were incubated in vitro with ground hay and increasing amounts of porcine bile acids or bile acid salts. Bile acids inhibited methane production up to 70% of the control incubation. The concomitant increase in propionate production compensated for the lower methane production so that the 2H-recoveries were in a normal range between 79-92%. Therefore the occurrence of reductive acetogenesis could be excluded. It is concluded, that bile acids are a controlling factor in caecal methanogenesis.