A brief description of some of the main physiological actions of serotonin has been given. Their clinical implications, especially where they pertain to the field of anaesthesia, are presented in some detail. The malignant carcinoid syndrome is discussed and some of the problems which may confront the anaesthesiologist in dealing with patients suffering from this condition are mentioned. I gratefully acknowledge the advice and assistance given by Dr. Donald E. Hale, Head of the Department of Anesthesiology, Cleveland Clinic Foundation, in the preparation of this paper.