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.