The tissue levels of S-Adenosylmethionine (SAMe) in 30-mo.-old rats were measured, and a remarkable decrease was observed compared to adult rats. The synthesis of SAMe by the methionine-activating enzyme and its utilization by COMT were investigated in different tissues. The activity of the synthesizing enzyme was unchanged in the liver and brain of adult and senescent rats, while COMT activity appeared to be higher in the aging rats. Thus, the result indicates that the decrease of same in these organs of senescent rats is due to the increased utilization rather than the decreased synthesis of this methyl donor compound.