Four Holstein cows with fistulated rumens were in a 4 X 4 Latin square design for us to determine effects of administering different sources of protein postruminally on milk production, milk composition, and efficiency of nitrogen utilization. Treatments consisted of postruminal infusion of 1) water, 2) sodium caseinate, 3) soybean meal, or 4) cottonseed meal. Milk yield (kg/day) and milk protein production (g/day) for the four treatments were: 1) 30.6, 1043; 2) 32.7, 1146; 3) 32.1, 1102; and 4) 31.1, 1087. Synthesis of milk protein increased during postruminal infusions of protein as indicated by the increased casein nitrogen fraction in milk. All protein infusions improved efficiency of use of absorbed nitrogen for synthesis of milk protein. Milk fat percentage and daily production of milk fat were not affected by treatments.