Only two S‐adenosyl‐l‐methionine synthetase forms exist in rat liver: high‐M_rS‐adenosyl‐l‐methionine synthetase and low‐M_rS‐adenosyl‐l‐methionine synthetase, which have been purified to apparent homogeneity as judged by sodium dodecyl sulfate/polyacrylamide gel electrophoresis. High‐M_rS‐adenosyl‐l‐methionine synthetase had an apparent molecular mass, determined by gel filtration, of 210 kDa and was a tetramer constituted by 48.5‐kDa subunits, estimated by sodium dodecyl sulfate/polyacrylamide gel electrophoresis. The apparent molecular mass of low‐M_rS‐adenosyl‐l‐methionine synthetase, as estimated by gel filtration, was 110 kDa and was constituted by two subunits of 47 kDa. An antiserum against low‐M_rS‐adenosyl‐l‐methionine synthetase cross‐reacted with the two forms. Reverse‐phase HPLC runs of tryptic digestions of high‐M_r and low‐M_rS‐adenosyl‐l‐methionine synthetase showed that the peptide maps of the two forms were very similar, if not identical. High‐M_rS‐adenosyl‐l‐methionine synthetase activity was inhibited by S‐adenosyl‐l‐methionine and pyrophosphate. Depending on the dose used, S‐adenosyl‐l‐methionine activated or inhibited low‐M_rS‐adenosyl‐l‐methionine synthetase and pyrophosphate had no effect on this form. The two synthetases showed a different specific activity at the physiological concentration of methionine. This report shows that even though the two forms are constructed of the same polypeptide chains, they are regulated in a different manner by methionine and by the products of the reaction.