The activities of S-adenosylmethionine synthetase isozymes in liver were measured after rats received a diet containing excess methionine. The activity of the α-form increased with increasing methionine content in the diet, and reached 4-5 fold after 6 days on a 3 methionine diet. However, the activity of the β-form showed only a 1.5 fold increase. The activity of the γ-form in kidney showed no significance change.