The relative contributions of sulfur atoms of dietary L-Cys and L-Met to the syntheses of proteins and GSH in rat liver were examined. 1) When the amount of sulfur-containing amino acids in L-Trp-deficient diets was fixed at 0.36%, incorporation of L-[35S]Cys into GSH was proportional to the amount of L-Cys administered, in the presence of L-Met in the diet. Incorporation of 35S from L-Met into GSH was lower than that from the same amount of L-Cys and became negligible in the presence of a large amount of L-Cys. 2) When rats were given L-Trp-deficient diet, more L-Cys was always incorporated into liver GSH than into proteins. But, when rats were given L-Trp-fortified diet containing more L-Met than L-Cys, more L-Cys was incorporated into liver proteins than into GSH. 3) Met-sulfur was preferentially incorporated into liver proteins with or without L-Trp. Its absolute incorporation into proteins was significantly greater in the presence of L-Trp than in its absence. 4) When the amount of L-Met in the diet was increased from 0.18 to 0.36 or 0.54%, incorporation of Met-sulfur into proteins increased proportionally, in the presence of 0.18% L-Cys. Unexpectedly, incorporation of L-Cys formed from L-Met into liver proteins was larger than that from L-[3H]Met itself. L-Cys formed from L-Met was incorporated into proteins more readily than L-Cys given as such. 5) When the amount of L-Met in the diet was increased from 0.18 to 0.54%, incorporation of Met-35S into GSH increased 8-fold. Even with a large excess of L-Met, L-Cys was invariably incorporated into GSH. 6) These results are consistent with the role of liver GSH as a reservoir of cysteine, as proposed by us.