The abdominal portion of the colon of 13 clinically normal dogs was divided into 5 regions (ascending, transverse, left colic flexure, proximal descending, and distal descending), and each region was injected with 30 mg of horseradish peroxidase (HRP). The injected colonic region, brain stem, L7-Cd1 portion of the spinal cord, sympathetic trunk ganglia, celiacomesenteric ganglia, caudal mesenteric ganglion, pelvic plexi, distal vagal (nodose) ganglia, and L1-Cd1 spinal ganglia were obtained at post-injection hour 48, sectioned, and processed by use of the tetramethylbenzidine method. The entire length of the colon was found to be under extrinsic influence of the parasympathetic nucleus of cranial nerve X (PX), with the largest average number of labeled cells resulting from injection of the ascending colon. It was also indicated that the entire colon is under extrinsic influence of the sacral portion of the spinal cord because the pelvic ganglia (second-order neurons) of the pelvic plexi contained labeled cells for all colonic regions. The largest average number of labeled cells in pelvic ganglia was seen after injection of the distal portion of the descending colon. Only after injection of the distal portion of the descending colon were labeled cells found in the S1-S3 portion of the spinal cord. Labeled cells in the PX, spinal cord, and pelvic ganglia were found bilaterally. Although the entire abdominal portion of the colon appears to be influenced by cranial and sacral parasympathetic preganglionic (via pelvic ganglia) neurons, the relative importance of the 2 areas seems to be reversed between the ascending colon and distal portion of the descending colon.(ABSTRACT TRUNCATED AT 250 WORDS)