The histamine and 5-HT contents of injured skin in rats are higher than in uninjured skin and indicate that mobilization of these amines from tissue mast cells occurs after aseptic injury. Healing of injured areas is greatly delayed in rats by depletion of skin histamine and 5-HT, but is enhanced by treatment with heparin, another constituent of the tissue mast cell. In the exudative and reparative phases of inflammation, depletion of histamine or 5-HT adversely affected the conditions. Both histamine and 5-HT appear to be involved in the animal''s defence against injury.