Adult rats may utilize two metabolites of methionine for the biosynthesis of this essential amino acid. In separate experiments methionine, labeled with 14C or with 35S was observed in plasma and urine following the administration of [2-14C]-alpha-amino-n-butyric acid or [35S]-5'-methylthioadenosine by stomach tube. Although alpha-amino-n-butyric acid (ABA) or homoserine, alone or with dietary sodium sulfate, choline, and/or S-methylcysteine, was not utilized for growth, weight loss in weanling rats was decreased by dietary cysteine when fed as an additive to a basal methionine-free, cysteine-free, labile methyl-free, sulfur-free diet. Following the addition of 10 mg ABA and 28 mg 5'-methylthioadenosine/day to the basal diet, growth response was equivalent to that occurring in rats receiving 27 mg of methionine/day with the basal diet. The implications of these findings for adaptation to protein restriction and a discussion of equilibrium and steady state conditions related to the increase in methionine content in the blood are presented.