While the formalin test is a widely used behavioral model of tonic chemogenic 
pain, little is known about the responses of primary afferent nociceptors to 
formalin. Formalin (2.5%, 50 microliters) was injected either directly in or 
adjacent to the mechanical receptive fields of single C-fibers isolated from the 
saphenous nerve of pentobarbital-anesthetized rats. The average formalin-evoked 
response in C-fibers (n = 29) over time was biphasic. This biphasic time course 
of the C-fiber response to formalin is similar to that of the behavioral 
response in the awake animal and is compatible with the hypothesis that 
increased C-fiber activity contributes to the behavioral response in phase 2, as 
well as in phase 1 of the formalin test.