1. Balance trials with respiration measurements were performed with twelve rats and twelve pigs given either low- or high-crude-fibre diets. There were six collection periods with the rats over a live-weight range of 86-264 g and three collection periods with the pigs over a live-weight range of 30-55 kg. Measurements were made on the influence of microbial activity in the digestive tract on digestibility and nitrogen and energy metabolism. Dietary inclusion of the antibiotic Nebacitin was the method used to reduce the microbial population. 2. The microbial activity in the hind-gut (mumol ATP/g air-dry contents) of antibiotic-treated rats was reduced to approximately one-tenth of that of untreated rats. 3. Live-weight gain was not significantly affected in either species by a reduction in the microbial activity, in spite of a reduction in dry matter digestibility in animals with reduced microflora. 4. For rats on low-crude-fibre diets, a reduction in microflora reduced digestibility of all nutrients and energy and metabolizability of digestible energy by approximately 5.4%. All differences were highly significant. On high-crude-fibre diets the decrease was approximately 5.9%. In pigs on both crude fibre levels, the digestibility was also influenced by the level of microflora, but the pattern was somewhat different from that obtained with rats, with the Nebacitin treatment increasing the digestibility of N slightly, and the digestibility of fat markedly. 5. Retained N in rats reached a maximum when the rats were approximately 60 d old and thereafter decreased with increasing age. However, for pigs daily N retention increased with age. The retained N: digested N value decreased linearly with age in the rats, but varied little with age over the range (104-146 d) studied in the pigs. 6. The metabolizability of gross energy (metabolizable energy (ME): gross energy) was significantly reduced with an increase in crude fibre level and by the addition of Nebacitin. 7. Retained energy (RE) in relation to ME (RE:ME), was not significantly affected either by level of microbial activity or by crude fibre. 8. The ratio, RE as fat (RF): RE as protein (RP) increased as the animals grew. In the rat experiment there was a tendency for RP to be higher for animals with normal microflora than for animals with reduced microflora for both crude fibre levels. 9. With rats, the regression analyses indicated that the energy requirement for maintenance could be influenced by both the level of microbial activity in the digestive tract and by the level of fibre in the diet. The net availability of ME for maintenance and growth by rats averaged 0.72 for all treatments. 10. The net availability of ME for growth in the pigs averaged 0.65 for all treatments.