Previous research at the Agricultural Research Institute of Northern Ireland has shown that equivalent milk yields can be obtained by feeding high levels of low-protein supplementary concentrate or low levels of high-protein supplementary concentrate. The latter may be due to an increase in dry matter intake and thus energy available for milk yield (an indirect effect) or an increase in amino acid supply from increased protein in the diet (a direct effect) or a combination of these. The experiment compared the effect of varying levels of protein supplement on rumen fermentation characteristics in lactating dairy cows.