Animal experiments are necessary to better understand the biological mechanisms of pain, and to develop new methods of pain therapy. Therefore, pain has to be produced experimentally in animals. The ensuing moral dilemma requires that the scientists involved have a high level of responsibility for the animal. A set of ethical guidelines has been developed by the International Association for the Study of Pain in order to minimize pain and suffering in such studies. Practically, this may be reached by, e.g. a careful design of the experiment or by analgesic treatment of the animal. The importance of reward in a behavioral paradigm involving pain is emphasized. Animal models of chronic pain will be necessary to understand the mechanisms of chronic pain in man. Observations of the behavior of animal patients should be used to define the species-specific behavioral expression of chronic pain. To observe only one behavioral parameter may be inadequate for the assessment of pain in animals. The training of the young scientist will be crucial to develop his general attitude of great ethical responsibility for the experimental animal and his conceptual and technical capabilities for optimal conductance of experiments. In practice, investigators engaged in research on pain in conscious animals should consider the following guidelines aimed at minimizing the pain in animals. The guidelines have been endorsed by the International Association for the Study of Pain. It is essential that the intended experiments on pain in conscious animals be reviewed beforehand by scientists and lay-persons. The potential benefit of such experiments to our understanding of pain mechanisms and pain therapy needs to be shown. The investigator should be aware of the ethical need for a continuing justification of his investigations. If possible, the investigator should try the pain stimulus on himself; this principle applies for most non-invasive stimuli causing acute pain. To make possible the evaluation of the levels of pain, the investigator should give a careful assessment of the animals's deviation from normal behavior. To this end, physiological and behavioral parameters should be measured. The outcome of this assessment should be included in the manuscript. In studies of acute or chronic pain in animals measures should be taken to provide a reasonable assurance that the animal is exposed to the minimal pain necessary for the purposes of the experiment.(ABSTRACT TRUNCATED AT 400 WORDS)