Four multiparous Holstein cows, averaging 44 days in milk and previously fitted with rumina1 and duodenal cannulae, were utilized in a 4 x 4 Latin square design to evaluate effects of dietary nonfibrous carbohydrate (NFC) and rumen undegradable protein (RUP) on metabolic and lactational parameters. Diets were isonitrogenous (17.4% CP) and structured in a 2 x 2 factorial arrangement with levels of NFC at 41 or 47% of dietary OM and RUP at 27 or 41% of dietary CP with inclusion of corn gluten meal, fish meal, and feather meal to increase dietary RUP. When cows were fed diets containing 47% NFC, intake of DM, OM, NFC, and N increased (P ± .05) while those containing 41 % NFC increased (P±.01) intake of NDF and ADF. Ruminal digestibility coefficients were not affected leading to greater quantities of NFC digested resulting in suppressed pH (P ±.01) and acetate (P ±.05) and increased propionate (P ± .05), Diets containing 27% RUP increased (P ±.01) ruminal ammonia and plasma urea nitrogen (N) with 41 % NFC eliciting higher ammonia N concentrations at each RUP level. Bacterial yields (averaging 220 g/d) were slightly higher when 41% NFC diets were consumed with no dietary effects on efficiency. Flow of ammonia N to the small intestine (SI) increased (P ± .01) with 27% RUP. When compared to 41% NFC, diets containing 47% NFC increased (P ± .05) flow of methionine (38.6 vs. 32.5 g/d). Fifty percent of the essential amino acids entering the SI were of bacterial origin. Diets containing 41% RUP elicited lower total tract digestion of N while enhancing that of hemicellulose. Total tract digestion of NFC and ADF was greater when 41% NFC diets were consumed. Yields of milk (38.7 vs. 37.7 kg/d) and lactose (1.94 vs 1.88 kg/d) increased with 27% RUP. Efficiency of production was increased by both 41% NFC and 27% RUP. Total and whey N in milk increased (P 5 ±.05) while milk urea N decreased (P ± .01) with higher (41%) dietary RUP. Increased (P ± .05) absorption of N (490.4 vs. 461.6 g/d) at the SI occurred with the 47 vs. 41 % NFC diets, respectively, with no effects observed on that retained or excreted in the urine or milk. Although supply of methionine to and the absorption of N from the S1 was greater with the 47% NFC diets, no positive effects were observed on nitrogen balance or milk production. Rather, dietary RUP provided greater promise in that lower dietary RUP increased milk production and efficiency while increased RUP enhanced milk N content. Therefore, economics regarding feed costs as well as price for milk and milk components may largely dictate preferential dietary RUP content. Abbreviation key: ADC = apparent digestion coefficient, CGM corn gluten meal, CRN = carbon hydrogen nitrogen, CHO = carbohydrate, CNCPS = Cornell Net Carbohydrate and Protein System, MUN = milk urea nitrogen, N = nitrogen, NAN = nonammonia N, NANMN = nonammonia nonmicrobial N, NFC = nonfibrous carbohydrate, NPN = nonprotein N, PUN = plasma urea N, RDP = rumen degradable protein, RUP = rumen undegradable protein, SI = small intestine, TDC = true digestion coefficient, 41:27 = 41% NFC and 27% RlUP, 47:27 = 47% NFC and 27% RUP, 41:41 = 41% NFC and 31% RUP, 47:41 = 47% NFC and 41% RUP. / Ph. D.