Human behaviour is defined as a cooperative phenomenon ('the whole is more than and different from a simple addition of its parts') in a hierarchical multi-component system, ranging over many subsystems from neuroreceptors to social interactions. Synergetics shows that in such a system there also exists a hierarchy of time constants ('reponse time'). The lower (chemical) subsystems have much smaller time constants than the higher (intraphysic or social) systems. For that reason rapid psychopharmacologically induced neurochemical alterations can be followed by changes in mood and reality testing much later. Full therapeutic response to lithium, antidepressants and neuroleptics may require 10 days to several weeks and may last for an equal period of time after cessation of therapy. The time-course of symptom relief can be quite different from the time-course of drug concentration in the body. Some practical guide-lines are given. Clinical consequences are discussed.