Responses to daily abomasal infusions of 400 g sodium caseinate, 400 g hydrolyzed casein or 11.3 g L-methionine-30.1 g L-lysine and rumen degradability of dietary protein were studied in eight Holstein cows during mid-lactation. Basal diets contained com silage, com and either soybean meal or 60:40 soybean meal:corn gluten meal, and had estimated rumen degradabilities of 70 and 60.5%. Duodenal cannulas were installed in four of the cows to allow measurement of digesta composition and flow. Methionine-lysine infusion increased milk protein percentage on both diets and milk fat percentage and yield on the soybean meal diet. Sodium caseinate increased milk and milk protein production, body weight gain, and decreased milk fat percentage, but not yield. Hydrolyzed casein did not produce the same responses, suggesting differences in amino acid absorption and utilization between the sources. Basal diet affected the responses to abomasal infusions demonstrating that amino acid nutrition of the cow was affected by dietary protein degradability. Concentration of total essential amino acids, branched-chain amino acids, and urea cycle amino acids were increased by the infusion of the caseins. There were differences between the caseins in their effects on individual plasma free amino acids. Methionine-lysine infusion increased plasma lysine and taurine, a metabolite of methionine, suggesting that infused methionine was extensively metabolized. Total duodenal nitrogen flow and non-microbial nitrogen flow tended to be increased by inclusion of corn gluten meal in the diet. Rumen degradation of crude protein was greater for the soybean meal diet. Both diets had lower rumen degradability than predicted from previous experiments. / Ph. D. / incomplete_metadata