Rat liver S-adenosylmethionine (AdoMet) synthetase appears as high-M(r) (tetramer) and low-M(r) (dimer) forms. Both are inhibited in the presence of GSSG at pH 8. The calculated Ki values are 2.14 and 4.03 mM for the high- and low-M(r) forms, respectively. No effect on enzyme activity was observed in the presence of GSH, but modulation of inhibition by GSSG can be obtained by addition of GSH. At a total glutathione concentration (GSH + GSSG) of 10 mM, a KOX of 1.74 was calculated for the high-M(r) form, whereas this constant was 2.85 for the low-M(r) AdoMet synthetase. No incorporation of [35S]GSSG was observed in either of the enzyme forms, and inhibition of enzyme activity was correlated with dissociation of both AdoMet synthetases to a monomer. The data obtained in the presence of GSSG seem to suggest that oxidation leads to the formation of an intrasubunit disulfide. The possible regulation of AdoMet synthetase activity by the GSH/GSSG ratio is discussed, as well as its in vivo significance.