1. An artificial rumen is described together with associated analytical and sampling procedures.
2. The equipment was used to study the effects of linseed oil fatty acids on the fermentation of sugar-beet pulp and of sucrose by rumen micro-organisms.
3. With sugar-beet pulp the addition of small amounts of linseed oil fatty acids had little effect on total gas production, utilization of sugar and the molar proportions of steam-volatile acids formed, but they strongly inhibited methane production, whereas large amounts of the fatty acids temporarily inhibited the production of gas and utilization of sugar.
4. With sucrose even the smaller amounts of fatty acids had a marked inhibitory effect on the production of gas and utilization of sugar.
5. The technique is assessed critically and the results are discussed in the light of previous experiments in vivo.