Axon reflex vasodilation following injury to the skin of the pinna of the ear was studied in rats by a thermometric method. Post-traumatic vaso-dilatation did not occur in animals treated with tyrosine ethyl ester, an inhibitor of chymotrypsin. Vasodilatation was not affected by treatment of the rats with chlorpheniramine (antihistamine) or cyproheptadine (antihistamine and anti-serotinin) or with aprotinin, soybean trypsin inhibitor, epsilon-aminocaproic acid or tosyl arginine methyl ester (inhibitors of trypsin and of some other proteinases). Taken in conjunction with the results of other investigations, these findings indicate that in the skin of the rat: (a) histamine and serotinin are not essential for the initiation of axon reflexes, and (b) the chymotrypsin-like proteinase of mast cell granules, released as the result of antidromic activity in sensory axons, may act as a kininogenase and be responsible for causing dilatation of arterioles at the efferent limb of the axon reflex.