Two feeding trials were conducted to study the effects of source of concentrate and level of concentrate and roughage on milk composition; changes in certain blood and rumen constituents; and efficiency of conversion of feed protein to milk protein. Trial A (63 days) employed two groups of six cows each. The following rations were fed: 1) Control consisting of 1 lb. concentrate per 3.5 lb. milk, 40 lb. corn silage. and orchardgrass hay, ad libitum; 2) Low-roughage composed of 5 lb. orchardgrass hay, 10 lb. corn silage, and concentrates ad libitum. Trial B (70 days) employed two groups (four each) of late and three of early lactation. A control ration similar to that fed in trial A, except 48 instead of 40 lb. of corn silage, and a low-roughage ration of only 2 lb. hay and concentrates ad libitum, were fed to two groups in each stage of lactation. The remaining early lactation group received a ration comparable to the control except the corn in the concentrate was replaced by lactose. The twelve early lactation cows were subjected to a balance study at the end of the feeding trial. Intake of concentrates and milk yields on the low-roughage rations were higher in trial A than trial B. Percent milk fat and the acetate/propionate ratio of rumen fluid were decreased by low-roughage; while rumen propionate was increased. No significant differences in milk protein were observed. Milk SNF and lactose-mineral were increased on low-roughage in trial B. Feeding only 2 lb. hay daily resulted in bloat and diarrhea; while blood glucose and gamma globulin were increased. Lactose increased rumen butyrate and tended to increase milk fat percentages above the controls. Digestibilities of all dry matter components, except crude fiber, were higher on low-roughage. Cows on low-roughage digested a larger proportion of the dietary protein, but were loss efficient in the conversion of digested protein to milk protein. / M.S.