A dual flow continuous culture fermentor system was used to estimate ruminal rate of protein degradation and influence of supplemental nitrogen (N) source on digestion of total dietary crude protein (CP), dry matter (DM) and organic matter (OM). Four experimental diets contained predominantly corn and were isocaloric (14.7 M J DE kg−1) and isonitrogenous (12% CP on a DM basis) with urea, soybean meal (SBM), linseed meal (LSM) or corn gluten meal (CGM) as sole supplemental N sources. Fermentor flow rates were adjusted daily to attain liquid and solid dilution rates of 10 and 5% h−1, respectively. Total dietary N digestibility was greater (P < 0.05) in fermentors receiving urea than in those receiving CGM, while fermentors receiving SBM or LSM were intermediate (72.9, 54.9, 64.0 and 63.2%, respectively). Bacterial CP flow was higher (P < 0.05) from fermentors receiving urea or SBM than from those receiving CGM, with fermentors receiving LSM intermediate (1.31, 1.29, 1.02 and 1.14 g d−1, respectively). True OM digestibility was greater (P < 0.05) for diets supplemented with urea or LSM than for those supplemented with CGM (65.1, 60.4 and 51.2%, respectively). The rate of protein degradation was greater (P < 0.05) for LSM-CP than CGM-CP with SBM-CP intermediate (15.73, 6.88 and 10.13% h−1, respectively). Calculated bypass of potentially digestible protein for SBM-CP, LSM-CP and CGM-CP was 15.9, 10.9 and 29.1%, respectively. Total amino acid flow (g d−1) was higher (P < 0.05) for fermentors receiving SBM than for those receiving LSM. Total bacterial amino acid flow (g d−1) was higher (P < 0.05) for fermentors receiving SBM than for fermentors receiving CGM. Differences among diets for dietary amino acid flow (g d−1) were not significant. Key words: Ruminal protein degradation, soybean meal, linseed meal, corn gluten meal, continuous culture system