The processes of aging result in many physiological changes which can lead to alterations in both pharmacokinetic and pharmacodynamic properties. Such changes can result in altered sensitivity to chemicals, whether drugs or environmental agents, in the elderly. It is extremely difficult, however, to generalize about the nature of such changes. Part of this may reflect disagreement, or at least, confusion in the literature over what the term 'age' means. It is not uncommon for a study to discuss age-related changes and, in fact, be comparing neonatal to pubescent or young adult animals. Therefore, it is clear that 'age' should always be clearly defined. When interest is in post-maturational organisms, and, at the very least, those at the mean life span of the population. The requirement has been stated before, but not always adhered to. Studies comparing only very young and terminally senescent animals may not provide fair representation of the effects occuring over the life-span of the species in question. While no broad generalization can be made about pharmacokinetic and/or pharmacodynamic changes, it is probably fair to state that such changes will often occur.