1. Endogenous amino acid losses in ileal digesta and excreta of adults cockerels fed on diets containing guanidinated forms of casein, soyabean meal and cottonseed meal were determined using homoarginine as a marker. 2. The ileal endogenous amino acid losses were markedly higher (P < 0.001) in birds given the cottonseed meal diet compared to those given the other two diets. The ileal endogenous protein was rich in aspartic acid, serine and glutamic acid. 3. Negative values were obtained for endogenous amino acid output in excreta. These aberrant values were caused by high concentrations of homoarginine in the excreta. A subsequent study with broiler chickens showed that the homoarginine in excreta was of urinary origin. 4. These results indicate that the homoarginine technique is not suitable for determining endogenous amino acid losses in excreta, but applicable when determinations are made in the terminal ileum.