1 An attempt has been made to monitor simultaneously the changes in blood flow, histamine content, histidine decarboxylase (HDC) activity and mast cell population in rat skin isografts and allografts. 2 The histamine content in rat skin allografts during rejection was decreased in contrast with our earlier observation in rabbits. 3 Although no definite correlation has been found between blood flow changes, histamine content or HDC activity, a reciprocal relationship appeared to exist between histamine content and HDC activity in the skin grafts, which might be a reflection of the immunosuppressive activity of this amine. 4 The immunosuppressive agent, cyclosporin-A (20 mg/kg daily) was able to prolong skin allograft survival and prevent the changes in histamine and HDC activity in allografts. 5 Possible implications of these findings are discussed in relation to current knowledge concerning the interactions between endothelial cells, lymphocytes and mast cells/basophils in graft rejection.