Rumen ammonia levels in animals given silage diets are generally high and there appears to be considerable scope for improving the utilization of ammonia for microbial protein synthesis (Thomas & Chamberlain, 1982). Chamberlain et al. (1985) showed that supplements of sucrose were more effective than supplements of starch in reducing the rumen ammonia levels in animals given silage diets. Moreover, the effects of sucrose were increased when it was given together with sodium bicarbonate. The implication was that the sucrose supplements increased the fixation of ammonia in microbial protein through the provision of additional fermentable energy, and that this process was enhanced by the sodium bicarbonate supplements. The experiment reported here was designed to provide quantitative information on these effects and in particular to elucidate more fully the effects of sodium bicarbonate supplements on nitrogen metabolism in the rumen.