1. It is shown that a basal roughage diet can be successfully supplemented with glucose, as long as nitrogen is not limiting. Lack of N depresses the digestibility of the basal ration, results in incomplete fermentation and the increased recoveries of N are consistent with fixation of atmospheric N2.
2. Using a complex, but soluble supplement (whey powder) it is shown that reproducible incremental measurements can be made and that the supplement used gives increases in production of characteristic end-products only (carbon dioxide, methane, acetic and butyric acids).
3. The location of the solid dietary components within the reaction vessel is not important and it is possible to measure changes in a particular component in the presence of others. It is shown that there is sequestration of bacteria and protozoa on the solid digesta.