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. The histamine content in rat skin allografts during rejection was decreased in contrast with our earlier observation in rabbits. 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. 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. 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.

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