The effect of dietary factors (usually controlled in practice) on microbial protein synthesis is reviewed using in vivo experiments. Attention is drawn on the necessity to clearly distinguish variations in microbial growth efficiency from those of intestinal flow of microbial protein and to consider simultaneously variations in feed protein degradation. In practice, the relationship between microbial protein synthesis and energy intake depends mainly on diet composition and the nature of the forage. Microbial protein flow to the intestine, relative to energy intake, is lower with high concentrate diets (when given in restricted amounts), with silages and with antibiotic supplements. This flow is increased by some forage processing (such as dehydration and alkali treatments), by natural or induced defaunation, and occasionally by increased feeding frequency (when intake is restricted) and buffer and vitamin supplements. However, with some factors such as feeding frequency and antibiotics supplementation, these variations are partly counterbalanced by reverse effects on feed protein degradation.