Subcutaneous injection of dilute formalin induces pain in humans and behaviors 
indicative of pain in animals. The formalin test, which is based on these 
observations, is now widely used as a model of pain produced by tissue injury, 
but the neural mechanisms of pain and analgesia in this test have not been 
identified. Rats with transections of the brain rostral or caudal to the pons 
show behavioral reactions to formalin similar to those of normal rats, although 
the temporary abatement of pain 10-15 min after formalin is absent in transected 
animals. Doses of morphine that suppress the behavioral response to formalin in 
normal rats are not antinociceptive in the formalin test in decerebrate rats 
although sedation, catalepsy and inhibition of the tail-flick reflex still 
occur. These results suggest that the response to formalin is organized in the 
brain stem but the antinociceptive effect of morphine in this test is mediated 
by the diencephalon or forebrain.