A fishmeal-based reference diet, a soybean-yeast-based methionine (MET)-deficient diet and two MET-supplemented diets were fed to groups of yearling sea-bass, reared in sea-water at 21 +/- 1 degrees C. Levels of most free essential amino acids (EAA) reached a maximum concentration in plasma between 4 and 8 hr after a single meal. Plasma MET level remained very low with the deficient diet, exhibiting by 4 hr a 10-fold reduction when compared to the reference diet, although MET content was divided only by two in the former. Daily MET intake was then maintained under minimum requirement in fish fed the deficient diet. Supplemented MET reached peak level in plasma substantially sooner than the rest of the EAA, indicating faster absorption and degradation when compared to MET incorporated in feed proteins. Evolution of methionine sulfoxide (MSO) levels in plasma after feeding MET-supplemented diets gives evidences that a degradation pathway of MET into MSO exists in sea-bass.