Two studies were conducted to evaluate the effects of varying dietary sources of rumen available carbohydrate (RAC) and rumen available protein (RAP) on milk yield and milk composition, nutrient flow to the duodenum, ruminal and total tract nutrient digestibilities, and ruminal pH, ammonia-N, and VFA concentrations in lactating cows. The first study was a response surface design utilizing nine dietary combinations of RAC and RAP. The response surfaces of all milk variables were saddle-shaped. Because of the saddle-shaped surfaces, an optimum combination of RAP and RAC for milk production variables was not obvious from the limited range of RAC and RAP used in this study. Ridge analysis of the saddle surfaces predicted maximum milk yield when dietary RAC was below 69 % of the DM and RAP below 60% of CP in alfalfa-corn silage based diets. In the second study, four cannulated (ruminal and duodenal) cows were utilized in a 4x4 Latin square design. Four of the nine original diets were selected to provide the largest range of RAC and RAP. Nutrient flow, digestiblities and ruminal parameters were evaluated. Although the in situ incubations indicated that rates of DM, CP, and NDF degradabilities differed among diets, no effects on overall ruminal pH and total VFA concentrations were detected. Additionally, DM, OM, NDF, ADF, and N flows to the duodenum were not affected by dietary treatment. Nonmicrobial N flow was greater for the barley-based diet, yet microbial flow was not different. The differences in rates of availability determined by in situ methods were not large enough to illicit a measurable difference in nutrient digestion and utilization. Additionally, the data implied that none of the diets were limiting in RAC and RAP for vigorous microbial activity. Fat-corrected (3.5%) milk production was greatest (P<.05) when alfalfa-corn silage based diets contained supplements providing intermediate (69 % RAe) carbohydrate availability (corn and barley) and low (60 % RAP) ruminal protein availability (BM and SBM). The increase in fat-corrected milk was consistent with the predicted milk production response in the previous study when RAP exceeded 62% of CP. However, the ruminal parameters, nutrient flow, and nutrient digestibility measurements did not adequately explain the increased milk production when diets contained increased concentrations of BM. / Ph. D.