Each of the four agents, ovomucoid, dextran, 48/80, and testis extract, when injected beneath the skin of the dorsa of the paws of rats produces a local vascular injury characterized by a protein-rich edema. Each agent also produces damage to mast cells. Either 5-hydroxytryptamine or histamine produces a response similar in the gross to that elicited by the agents which damage mast cells; however, neither of these two agents produces mast cell damage. On a weight basis 5-hydroxytryptamine is a much more potent edema-producing agent than histamine. The edema-producing action of 5-hydroxytryptamine can be differentiated from the similar action of histamine by the use of specific antagonists; dibenamine is a 5-hydroxytryptamine antagonist and pyrilamine a histamine antagonist. The edema produced by the mast cell-damaging agents is partially inhibited by dibenamine but is not diminished by pyrilamine. It is completely inhibited by treatment of rats with both drugs. The drugs which inhibit edema do not prevent mast cell damage by ovomucoid, dextran, 48/80, or testis extract. The observations are consistent with the hypothesis that agents which damage mast cells, "release" both 5-hydroxytryptamine and histamine and that in the rat the edema associated with mast cell damage is mediated largely by 5-hydroxytryptamine.