Wherever the age of an individual can, of course, be precisely indicated, the individual physiological state of every aged person varies greatly and therefore the resultant implications considering absorption, distribution, metabolism and excretion. In animals aging has been described as a factor of modification of metabolism. In the elderly the oxidative enzyme activity appears diminished but coupled to a decreased liver mass and liver blood flow. Examples of decreased conjugation for paracetamol and indomethacin are known. Acetylation does not seem to be influenced. The net effect of aging appears quite frequently an increase in the blood level of unchanged drug. However, drug metabolism cannot be considered independently from the other factors calling for an individualized treatment of the elderly.