I have tried to convey the notion that the pain system is a very flexible one. There is considerable excitement at present that we are beginning to understand the mechanisms whereby this flexibility results in clinical pain states; this must be a good prelude to trying to prevent or reverse them. But what is the relevance of all this to pain states in animals? There are still fundamental aspects about which we know too little. Detection of pain in animals is still difficult, and even when pain is present we often cannot tell how much it is affecting the animal, especially under conditions of chronic disease. We still do not know enough about the efficacy and kinetics of analgesic drugs in each species, a task for pharmacologists and clinicians alike. We know that sensitisation of nociceptive systems can occur after tissue trauma, as occurs during surgery, but we do not yet know whether this can be effectively prevented. Against this background of tasks yet to be undertaken, we can be optimistic that we are on the brink of understanding the triggers that alter pain sensitivity, that we shall soon know which mediators are involved, and that this will give rise to new strategies for the prevention as well as the alleviation of pain. There is indeed a period of excitement ahead. In all of this we must not lose sight of the ethical and welfare considerations of using animals for pain research.(ABSTRACT TRUNCATED AT 250 WORDS)