AbstractThe body is considered as a system composed of a number of subsystems. The response(s) of a drug is a complicated function of the concentration in the blood plasma, which in turn is some function of the dosage input.The dose-response curve of a drug in a subsystem (e.g., isolated organ) is, over a limited concentration range, a linear function of the logarithm of the concentration. The logarithm of the concentration in the plasma is, again over a limited range, a linear function of time. Time-effect curves in the intact organism may therefore be a linear function of time.In reality, the situation is far more complex, because of nonlinear kinetics, nonlinear kinetics of effects in the subsystems, and adaptation phenomena based on feedback regulation, as is illustrated by examples.It is concluded that one should not only consider the body as a system but also study it as a system; that is, apply the dynamic system approach in pharmacology.