Abstract Five cereal grains were each fed at 3 levels (10, 20, 30 g) to starved adult cockerels and the excreta voided during the subsequent 24 hr were collected; a group of unfed birds provided metabolic plus endogenous excreta. The difference between the input and output of amino acids and energy increased in a curvilinear manner with the level of input. When correction was made for metabolic plus endogenous excretion the variability between input levels became insignificant. The corrected differences are estimates of the true available amino acids.