Since the classical work by Trendelenburg (1917), many studies have been published on the peristaltic activity of the small intestine (for references see Builbring & Lin, 1958). In a recent investigation Biilbring & Lin (1958) found that small amounts of 5-hydroxytryptamine (5-HT) were continuously released into the lumen of an isolated loop of ileum and that 5-HT, if applied intraluminally, stimulated peristalsis by its action on sensory receptors deeply seated in the intestinal mucosa. These findings have been extended by experiments on small intestine in situ (Bulbring & Crema, 1958, 1959a, but little is known about the effect of 5-HT on, and the release of 5-HT from, the large intestine. Moreover it is not known whether the release of 5-HT from the intestine is controlled by the extrinsic nerves. Bayliss & Starling (1900) have described the movements of the large intestine in situ and the effects of stimulating its sympathetic and parasympathetic nerve supply. Since the distal colon can be excised with both the lumbar sympathetic outflow and the pelvic nerve attached (Garry & Gillespie, 1954, 1955, it seemed of interest to study the action of 5-HT on the peristalsis of the isolated colon and to investigate the effect of extrinsic nerve stimulation on 5-HT release.The effects of atropine, of hexamethonium and of nicotine on the peristaltic reflex as well as on the extrinsic nerve stimulation were also investigated, in the hope of throwing some light on the relationship between the nervous pathways of the peristaltic reflex arc and those of the extrinsic innervation.A preliminary account of some of the results has been given in a communication to the Physiological Society (Lee, 1959).