S-Adenosylhomocysteine, a potent intracellular methylation inhibitor, is suggested as a potential mediator for hyperhomocysteinemia-related vascular changes. We investigated the effect of acute and chronic hyperhomocysteinemia on intracellular S-adenosylhomocysteine and S-adenosylmethionine in rats and humans. Elevated plasma homocysteine in rats infused with homocysteine produced an increase in S-adenosylhomocysteine (P < 0.001) but not S-adenosylmethionine levels (P > 0.05) in various rat tissues. However intraerythrocyte S-adenosylhomocysteine and S-adenosylmethionine levels were not changed in homocysteine-infused rats and human subjects with experimentally acute hyperhomocysteinemia by methionine loading test. In contrast, erythrocyte S-adenosylhomocysteine levels were significantly higher in chronic renal failure patients, who had chronically elevated plasma homocysteine levels, than in either vascular disease patients or healthy controls (P < 0.05). In conclusion, acute hyperhomocysteinemia can increase intracellular S-adenosylhomocysteine levels in tissues actively involved in homocysteine metabolism. The findings are relevant to homocysteine-related endothelial dysfunction since S-adenosylhomocysteine modulates endothelial cell apoptosis.