Carrageenan was subcutaneously injected in the area innervated by the saphenous nerve. Part of the mechano-heat sensitive C-fiber receptors (CHM) located inside or at the border of the inflamed area showed an enhanced responsiveness to heat stimulation (sensitization). Those CMH units exhibited spontaneous activity; their mechanical thresholds (von Frey) were higher than those of not spontaneously active fibers. None of the units located outside of the inflamed area displayed sensitization. The data reveal that only part of the CMH units in a uniformly inflamed skin area shows signs of sensitization. Our results are compared to those obtained in other inflammatory processes. The relation to inflammatory pain and to hyperalgesia and the contribution of endogenous substances to sensitization of CMH units are discussed.