Every dermatologist has heard about histamine, but not every worker in this field of medicine has come across 5-hydroxytryptamine (5-HT). This is an amine related to, and probably derived from, the amino acid tryptophan. The formulae of these two substances are shown in Figure 1, together with that of the inactive excretory product of 5-HT, namely, 5-hydroxyindoleacetic acid (5-HIAA). The following pages indicate why the release of 5-HT should now be considered as part of the system concerned with the reaction of the skin to injury and with the subsequent formation of edema. Up to the present time, 5-HT has been found in all tissues containing cells belonging to the enterochromaffin system, in the blood, in the spleen, and in some central nervous structures. Its physiological significance, however, still remains unsolved, though many possibilities exist; it may influence hemostasis, or control vascular tone and