Thirty six Holstein cows in their first, second, third or fourth lactation were used in 2 x 2 factorial design to evaluate nitrogen (N) and carbon (C) partitioning to milk, urine, feces, and body tissue during early and midlactation. Diets containing 16% CP were formulated with 30 or 39% rumen undegradable protein (RUP) obtained by substituting blood meal (EM) for soybean meal (SBM). Each level ofRUP was formulated with supplemental phosphorus from mono- and dicalcium phosphate or wheat bran. Dry matter intake was higher in midlactation compared with early lactation, and increased as parity increased. Addition of BM to the diets decreased milk protein percentage and yield compared with SBM. Fecal N excretion was higher for cows fed BM due to lower N digestibility (67 versus 63%). Cows fed SBM retained more N and partitioned more N into milk than cows fed BM. Cows partitioned approximately 49, 40, and 11% of absorbed N to urine, milk, and tissue, respectively. Concentration of plasma urea N was correlated with milk urea N (r = .50). Overall, data indicated that cows fed 16% dietary CP with SBM or BM met their requirements for milk and tissue protein synthesis. Carbon partitioning was very similar to N partitioning in response to parity. Using a fermentation balance equation, it was estimated that .3 and 3.1 kg C were partitioned daily to methane and carbon dioxide, respectively. Estimated data indicated that 36% of intake C and 57% of absorbed C were lost to the atmosphere as carbon dioxide plus methane.