The effect of adding cottonseed hulls to casein- and cottonseed-kernel-based diets on the apparent and true ileal digestibility of N and amino acids, and the proportion of this effect accounted for by condensed tannin (CT), were determined using the growing rat. Sixtyrats were allocated randomly to ten semi- purified diets, containing either casein (four diets) or purified unheated solvent-extracted cottonseed kernel (six diets) as the sole protein source, with Cr2O3 added as an indigestible marker. Two of the casein diets contained no hulls whilst the remaining two diets contained 70 g cottonseed hulls/kg. Two of the cottonseed-kernel-based diets contained no hulls, with two containing 23 g hulls/kg and the remaining two containing 46 g hulls/kg. For each pair of diets, PEG was either included or excluded. The effect of CT was quantified by comparing control rats (-PEG; CTacting) with PEG-supplemented rats (+PEG; CT inactivated) at each level of dietary hulls. The rats were given their respective experimental diets for 14 d. Each rat was given the food ad libitumfor 10 min hourly from 08·00 to 18·00 hours. On day 14, samples of digesta were collected at death from the terminal 150 mm of ileum at 7 h from the first meal. Apparent and true ileal digestibilities were calculated for DM, N and the individual amino acids. The principal finding was that the inclusion of hulls depressed the apparent and true ileal digestibilities of N and amino acids, but with the response differing between diets. With the casein-based diet the mean apparent and true ileal amino acid digestibilities were significantly depressed from 0·89 and 0·96 to 0·85 and 0·92 respectively, by the inclusion of 70 g hulls/kg in the diet, and addition of PEG then restored these to 0·89 and 0·95. All of the depression could be explained by the CT content of the hulls. However, with the cottonseed-kernel-based diet the responses fell into three categories. The apparent and true ileal digestibilities of the essential amino acids cystine and methionine were not affected by hull addition, ileal digestibilities of leucine, isoleucine, lysine, threonine and valine were markedly depressed by hull addition with approximately 50% of the depression being explained by CT, whilst the ileal digestibilities of histidine, arginine and phenylalanine were depressed by hull addition but little or none of this effect could be explained by CT. Thus the effect of hulls on protein digestion clearly differed with source of protein. With the cottonseed-kernel-based diet it seems that components of the hulls other than CT also depressed the apparent and true ileal digestibilities of N and amino acids. The identity of these components is unknown.