The entire bowel of a mutant strain of rats, the congenital aganglionosis rat (spotting lethal), was investigated using the acetylcholinesterase reaction and immunohistochemical staining for tyrosine hydroxylase and substance P in whole-mount preparations. The histology of the bowel of mutant rats was also studied by light- and electron microscopy. In all examined mutant rats, a constricted region of intestine followed a dilated region of the bowel. In 29 cases constricted segments extended from rectum to distal ileum; in 3 cases from rectum to middle colon. In controls the myenteric plexus appeared as a mesh-work consisting of ganglion strands and internodal strands, showing a rather regular ladder-like pattern from duodenum to rectum. The myenteric plexus of mutants was very different from that of controls, showing conspicuous regional differences. Even in the duodenum, where there was no macroscopical disorder, the plexus showed an irregular pattern, the meshes varying greatly in size and shape. Ganglion strands were shorter than those in controls. The plexus in the dilated segment gradually decreased in density, finally disappearing above the proximal terminal of the constricted segment. In some areas below this transition, i.e., the anal portion of the constricted segment in 29 cases (long constricted segment type only), there were neither ganglion cells nor nerve fibers except for scarcely distributed tyrosine hydroxylase-immunoreactive nerve fibers. In the distal part of the upper colon some fine nerve bundles appeared to run irregularly. These nerve bundles gradually increased in number and mixed with thicker nerve bundles in the lower portion of the colon. Finally, at the level of the rectum, nerve bundles of various sizes interlaced irregularly with one another to form a network. However, this network was free from ganglion cells.