Accumulation following repeated doses is no individual behavior of a drug, but always happens if the rate of invasion of a drug exceeds its rate of evasion. Using parameters such as dosing interval and biological half-life of a drug, its rate of accumulation as well as the equilibrium drug concentration in the body can be calculated. The initial dose and the maintenance dose can be calculated in a similar way. For an individual case the reliability of data calculated on the basis of mean values for biological halflife may be limited by the statistical variation of means and by factors altering the elimination of the drug, e.g. pathological changes or the influence of drugs given additionally. Nevertheless, these data may serve as a guidance to the physician to help him find the individual dosage for his patient.