Patients with unstable scars in some areas of the body complain of local itchiness and pain, which may be caused by histamine and serotonin. Therefore, I measured these substances in scar tissue in various conditions taken at the time of operation. The histamine value was determined by the amount of carbon dioxide produced in histidine decarboxylase activity (HDCA); the measurement of 5-hydroxytryptamine (5-HT) was done by the Ninhydrin method. High values of HDCA were usually found in skin flaps and scar tissue around pressure sores. 5-HT activity reached a higher level in unstable scars than in stable scars.