Increasing feeding costs has led many dairy farmers to use pasture for lactating cows to reduce their production cost. Little is known about the efficiency of nutrient utilization by lactating cows under grazing conditions. The objective of this study, therefore, was to investigate ruminal turnover, intake and site and extent of nutrient digestion in grazing lactating cows. Four dual cannulated (rumen and proximal duodenum) cows were randomly assigned to two groups to graze permanent pasture with no supplement or with 6.4 kg/d corn-mineral mix in a switchback design with three periods of two wk each. Solid and liquid markers (chromic oxide and CoEDTA), used to estimate duodenal flow, fecal output, and ruminal turnover, were administered through the rumen cannula twice daily at 1100 and 2300 h. The supplemented cows had greater milk production (23.7 kg/d) than those on pasture only (19.5 kg/d) with an increase of .66 kg of milk production per kg of concentrate. Corn, however, depressed milk fat percentage resulting in a similar milk fat yield between the two diets. Supplemental corn reduced rumen ammonia-N (22 vs 17 mg/dl) and increased N recovery at the duodenum (86 vs 75% of N intake). True Nitrogen digestibility averaged 72% and microbial N flowing to the duodenum was 67% of the total N flow. This results indicated that grazing cows may benefit from concentrates containing undegradable protein. Daily OM intake was not different, but when cows were fed the supplemented diet, pasture OM was lower than when fed pasture only. Organic matter, NDF and ADF digestibility in the rumen and whole digestive tract were greater when cows were fed. Cellulolytic activity may have been reduced by grain supplementation due to decreased ruminal pH (6.4 vs 6.2). The reduction in fiber digestibility may explain the decrease in forage intake when cows were fed supplemental grain. Ruminal rates of passage (kp) for solid (7.3 %/h) and liquid (18.2 %/h) markers were similar for both diets. The kp values observed for solid and liquid indicate that grazing cows may have a faster ruminal turnover than cows fed diets containing primarily hay or silage. / Master of Science