We used a newly developed procedure to determine reduced, oxidized, and protein-bound forms of homocysteine, cysteine, cysteinylglycine, and glutathione to measure the plasma concentrations of these species during methionine loading in six young healthy men with normal fasting concentrations of plasma homocysteine and cysteine. The methionine loading induced a transient increase in total homocysteine, which peaked after approximately 6-8 h. All six subjects showed a concurrent significant increase in reduced homocysteine and cysteine, which peaked 2 h after loading, and a rapid decrease in protein-bound cysteine and cysteinylglycine. The concentration of reduced cysteinylglycine was not altered. Plots of protein-bound cysteine and cysteinylglycine vs total homocysteine formed hysteretic loops, showing a time-dependent relation between these analytes. After the initial decrease, protein-bound cysteine and cysteinylglycine showed a slight, transient increase. From 12 to 24 h after loading, protein-bound cysteine approached preloading concentrations in two subjects and declined further in four subjects. The response pattern was similar for cysteine and cysteinylglycine in each subject. Simple displacement could not account for these effects, which suggests that plasma homocysteine may affect the disposition of other thiols through complex mechanisms. The presence of reduced homocysteine and the dynamic relation that exists between homocysteine, cysteine, and related compounds in plasma should be taken into account when evaluating plasma homocysteine as an indicator or causative agent of human disease.