The formalin test has been used in monkeys for assessing pain. After formalin injection in the palmar surface of the hand just proximal to the base of the fingers, the monkey's responses are rated for 1 h according to objective behavioral criteria. The present 'tonic' pain model has a fair degree of objectivity, validity, reproducibility and quantifiability. The analgesic effects of morphine and pethidine have been evaluated.