The effect of consuming concentrated rations supplemented or not with bovine tallow on pH, ammonium nitrogen concentration, protozoa populations and efficiency of nitrogen compound synthesis by microorganisms of the bovine rumen were determined in two experiments. In both studies, offering the animals corn silage combined with concentrates containing 4 to 10% bovine tallow caused a reduction in ammonium nitrogen and in ruminal protozoan populations. The daily production of microbial mass per gram total digested carbohydrate in the rumen was 0.416 g in the absence of tallow, 0.607 g in the treatment containing 4% tallow, and 0.790 g in the treatment containing 10% tallow. The estimate of pH in the ruminal fluid and of the efficiency of synthesis of microbial nitrogen compounds obtained with the ruminal submodel CNCPS (Cornell Net Carbohydrate and Protein System) was found to be compatible with in vivo observations.