The metabolism of sulfur of S-methyl-L-cysteine and ($)S-methyI-L-cysteine sulfoxide in Chinese cabbage was studied. Results showed that these sulfur-bearing amino acids are metabolized to cysteine by demethylation. This conclusion is based on; 1) the relatively high recovery of radioactivity in the cysteine of the insoluble fraction a short time after the administration of 35S-labeled S-methyl-L-cysteine or ($)S-methyl- L-cysteine sulfoxide; 2) the similarity in the ratios of 3H to 35S in the cysteine of the insoluble fraction and in the double-labeled cysteine part of S-methyl-L-cysteine and its sulfoxide fed to detached leaves and 3) the obvious structural relationship between cysteine and its methyl derivative, S-methyl-L-cysteine. Moreover, we ascertained that the sulfur of S-methyl-L-cysteine was also utilized when supplied to plants; i.e. tomato, tobacco and cucumber, in which this sulfur amino acid has not been found.