A rational approach to drug therapy in the elderly requires an understanding of the changes in phannacodynamics (drug response) and phannacokinetics (drug handling) that are likely to occur in this age group. The two are essentially interrelated since phannacodynamics, which may in a general sense be detennined by the amount of active drug available at the site of action (be it a physiological function or a pathological process), will obviously be influenced by phannacokinetic changes in the elderly. Alterations in the sensitivity or density ofthe receptors on which drugs act or in the integrity of the physiological homeostatic mechanisms are further factors that, together with the presenceof disease, are likely to influence phannacodynamics in old age.