SYMPTOMS of anaphylaxis and the relative importance of individual chemical mediators vary widely in different species. The importance of histamine has been established in the guinea-pig1 and the dog2, while 5-hydroxytryptamine has been shown to play a principal part in anaphylactic reactions in the rabbit3. Bradykinin4 and a slow-reacting substance of anaphylaxis (SRS-A)5 have both been found to be involved in anaphylaxis in the guinea-pig6 and SRS-A in the rat7.