Pain has been defined by the International Association for the Study of Pain as “an unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage”. However, the ability to describe the concept of pain is difficult largely because pain is an individualized and subjective experience. The mechanisms of nociception in animals have been found to resemble closely those present in humans. Although the exact nature of the experience of pain in animals may well differ from that experienced by humans, it is obviously an unpleasant sensation which all animals will try to avoid. Animals in pain show considerable alterations in physiological variables and also deviation from their normal behaviour. The problem of our inability to assess pain accurately in animals is almost certainly the major reason for our failure to control postoperative or other kinds of pain effectively. It has also been proved difficult to quantify pain in humans. Analgesia is not, however, refused to human patients purely because quantification is unreliable and subjective. Similarly, in animals the responsibility to relieve pain should not be avoided because pain cannot be accurately quantified. It is recommended that researchers should attempt to assess pain in experimental animals using an appropriate quantitative pain-scoring system, by selecting the variables used for scoring to suit the specific animal model concerned. The choice of variables should be made after observing a small number of animals in a pilot-study and determining which variables are most affected by the procedure. Relieving the pain inflicted on experimental animals by a research protocol is not just a moral obligation; it is also a scientific requirement, since pain may adversely affect research outcomes.