The extent of ruminal protein degradation of 11 energy sources was determined in the rumen of cannulated lactating dairy cows, and was calculated at three different fractional outflow rates, using the in situ polyester bag technique. The extent of ruminal protein degradation ranged from 45% for bird-resistant sorghum to 91% for oats, when calculated at a fractional outflow rate of 0.08 / h, indicating large variation in the resistance of energy sources to protein degradation in the rumen. Among the energy sources, sorghum, maize and maize com and cob meal were noteworthy in providing larger amounts of undegraded dietary protein (UDP).