1. The paper describes the development and construction of an apparatus for maintaining a normal microbial population of the rumen under strictly controlled conditions over long periods of time. 2. The apparatus is simple to construct and operate. It is possible to do four replicate experiments at the same time. 3. The results of three experiments are given. The experiments showed that when the steady-state was reached it could be maintained indefinitely, with the type and quantities of products of fermentation very similar to those in the rumen of donor animals, including the maintenance of normal protozoal populations for up to 49 d. 4. It was found that within wide ranges, the digestibility of rations and the output of products were independent of dilution rate. 5. Except for the lowest 'level of feeding', the digestibility was independent of the level of feeding. The output of products was proportional to the amount of food digested and was the same as would be expected in sheep on similar rations. 6. An experiment in which a ration of hay was changed to a mainly concentrate ration showed that the fermentation characteristics were determined mainly by the food given.