Under the 1876 Cruelty to Animals Act it is necessary to recognise pain so that an assessment may be made to determine if it is 'an experiment calculated to give pain' and 'to prevent the animal feeling pain'. Under the conditions of the licence it is also necessary to recognise 'severe pain which is likely to endure' and 'suffering considerable pain'. In the White Paper May 1983 (Command 8883) it is stated that: 'in the application of controls the concept of pain should be applied in a wide sense' and 'the Home Secretary's practice has been to interpret the concept of pain to include disease, other disturbances of normal health, adverse change in physiology, discomfort and distress'. The draft European Convention for the Protection of Vertebrate Animals used for Experimental and other Purposes, aims to control, subject to specific exceptions, any experimental or other scientific procedure which 'may cause pain, suffering, distress or lasting harm'. (The White Paper states that UK control will be stricter than the Council of Europe proposals.) Thus, there is a considerable onus on the experimenter to recognise pain (not to define it) and to alleviate it. It is intended that this article should be of help, not only to newcomers inexperienced in the recognition of pain, but also possibly to those relatively experienced workers who may be called upon to evaluate the pain involved in a new model or an individual animal.(ABSTRACT TRUNCATED AT 250 WORDS)