In part 1 of this experiment, six lactating Holstein cows were used to determine rate of passage of soybean meal, corn gluten meal, and brewer's dried grains through the gastrointestinal tract and also to compare chromiumethylenediaminetetraacetic acid and cobalt-ethylenediaminetetraacetic acid as liquid digesta markers. Cows were offered a basal diet containing 60% alfalfa hay and 40% of a grain mix twice daily. In addition, all cows received 2.7 kg daily of a mixture of soybean meal, corn gluten meal, and brewer's dried grains. At a designated feeding, each of the protein supplements was labeled with a separate rare earth marker element (samarium, cerium, or lanthanum). Mean ruminal turnover of liquid was 11.4 and 9.9%/h when chromium-ethylenediaminetetraacetic acid and cobalt-ethylenediaminetetraacetic acid were used as markers. Treatment means for ruminal turnover of soybean meal, corn gluten meal, and brewer's dried grains were 4.9, 5.3, and 4,8%/h. In part 2, nitrogen disappearance from soybean meal, corn gluten meal, and brewer's dried grains placed in Dacron bags suspended in the rumen of a fistulated cow was determined at intervals of 1, 4, 8, 12, 16, and 24 h. These measurements, along with information on the amount of protein supplement remaining in the rumen at various intervals (derived from the rate of passage data), were used to estimate protein degradability in the rumen. Protein degradabilities by this approach for soybean meal, brewer's dried grains, and corn gluten meal were 72, 42, and 14%. Results for soybean meal and brewer's dried grains matched in vivo results in the literature, but corn gluten meal was underestimated grossly.