In ruminant feeding, the prediction of metabolizable protein requires accurate estimates of feed protein degradation in the rumen. Some common feed protein sources have a high concentration of neutral detergent insoluble nitrogen (N-NDF) whose degradation may be dependent on neutral detergent fibre (NDF) degradation and, consequently, on rumen cellulolytic activity. This may lead to an underestimation of the undegraded protein that reaches the small intestine, particularly when animals are fed high starch diets. The objective of this study was to verify if there is a relationship between the ruminal degradability of the NDF and N-NDF of eight feed supplements currently used in ruminant feeding with the in sacco technique.