Pain behaviour can be understood as a reaction to a noxious stimulus, leading to elimination of that stimulus. This behaviour is of vital importance. A noxious stimulus leads to local production of substances, like prostaglandins, that produce a local reaction and initiate conduction to the spinal cord. Analgesics like aspirin inhibit prostaglandin synthesis. Different prostaglandine have different important functions in various organs. Therefore analgesics may interfere with many important local defense mechanisms. The stimuli are conducted by special fibers to the CNS. On the level of the spinal cord they are already modulated by incoming sensory stimuli and by impulses from higher centers in het CNS. It is possible that stimulation of sensory nerves has an analgesic effect on this level. At higher levels in the CNS modulation also occurs. Endorphins play an important role in this modulation. Opiates bind to endorphin receptors and thereby modulate pain. However, endorphins are also presents in other organ systems, where their role is still poorly understood. Possibly their function is to terminate reactions to noxious stimuli that in themselves have become too strong.