Thirty primiparous Holstein cows in mid lactation (106± 24 d) were randomly assigned to one of six diets in an incomplete block design. Each cow received two of six diets. Cows received the first diet for 32 d, then were offered a different diet for 32 d. received two of six diets. There were 5 observations per diet per period for a total of ten observations per treatment. Diets were 12% CP soybean meal (LSB), 20% CP soybean meal (HSB), 20% CP fishmeal (FM), 20% CP corn gluten meal (CG), 20% CP CG:SB, and 20% CP FM:SB. All diets were > 75% TDN and ~ 16% acid detergent fiber. Milk yield was not different between treatments. Milk fat percent, milk lactose percent, and fat corrected milk yield were higher in cows on CG (3.13%, 5.30%, 23 kg) in contrast to cows receiving FM (2.11%, 5.00%, 20.5 kg). Milk protein percent was not different, but milk protein yield was different across treatments. Rumen ammonia concentrations differed significantly with HSB (13.5 mg/dl) higher than LSB (7.33 mg/dl), indicating the increase in ruminally degraded protein. Plasma urea concentrations reflected rumen ammonia concentrations with HSB (16.2 mg/dl) higher than LSB (6.4 mg/dl). Results suggest that protein source can affect milk composition in cows fed low fiber diets. / Master of Science