The intramural pelvic nerves of the dog colon were studied morphologically and electrophysiologically. These nerves emanate from the pelvic plexus, and ascend between the muscle layers along 57.5% of the colon's length. Many thin branches connect with Auerbach's plexus. Most of the nerves responded bilaterally with compound action potentials of A delta and C fibres to stimulation of the sacral dorsal roots, and with those of C fibres to stimulation of the ventral roots. Mean conduction velocities of the A delta and C fibres were 9.0 and 0.9 m/s, respectively. Distension of a section of colon at up to 85.0% of the length of the colon from the anus activated afferent fibres, and electrical stimulation of points at up to 102% of this length elicited responses in efferent fibres. In dogs in which the continuity of the colonic wall was interrupted by a ligature but the nerves were left intact, the intramural pelvic nerves were found to convey centrifugal activities of the recto- and ano-colonic reflexes to the proximal colon, and centripetal and centrifugal activities of the colo-colonic reflex.