Incubation inside the bovine rumen.

I . Details are given for the construction and use of an apparatus for the incubation of rumen liquor in semi-permeable sacs inside the bovine rumen. It was found practicable, using one animal, to incubate simultaneously samples of strained diluted rumen liquor in three identical vessels, each of 100--120 ml capacity, for periods of up to 23 h. 2. The optimal conditions for experiments of this type were investigated. It was shown that, under the conditions used, the pH value of the reaction mixtures varied little compared with the pH changes that occurred inside the rumen, and that the sampling technique was satisfactory. 3. The effects of the size and type of inoculum were also investigated. The microbial growth, measured by increases in turbidity and in concentration of protein, was greatest when the rumen liquor inoculum constituted about 20% of the reaction mixture and when the protozoa and large food particles were removed. 4. The apparatus was used to study microbial growth and hydrogenation of fatty acids in rumen liquor.