Abstract Mature sheep, fitted with cannulae in the rumen and small intestine, were fed diets of whole roughage or roughage-mixed concentrates. From the quantities of amino acids passing into the proximal duodenum and terminal ileum, the true digestibility of the individual amino acids was estimated by multiple regression. The overall true digestibility of amino acids was 85% (SEM 1.8%). Values obtained varied from 72% (cystine) to 93% (arginine). Apparent digestibility of amino acids, as determined by true digestibility and ileal endogenous loss, was positively related to the quantity of protein, but negatively related to the quantity of non-protein dry matter reaching the duodenum. Practical feeding implications under temperate and tropical conditions are discussed.