Summary. Histamine concentrations in plasma, whole blood and various tissues of human subjects, monkeys, pigs and dogs were determined by fluorometric methods before, during and after surgical operations. Following intraabdominM surgery in 6 of 22 patients elevated plasma histamine levels were found several hours after the end of operation. Some of the causes of histamine release in surgery were found to be premeditation by atropine, intravenously administered anaesthetics, infusion of plasma substitutes and manipulation on the gut. Acute blood losses were without effect on the plasma histamine levels. Clinical symptoms and pathophysiological reactions, such as taehyeardia, hypotension, increased gastric secretion and anaphylactoid reactions could be related to the release of histamine in some circumstances.