The rabbit appendix, a region of gut with well organized zones of lymphoid tissue, was examined with the glyoxylic acid histofluorescence technique for the localization of noradrenergic fibers, with high performance liquid chromatography with electrochemical detection (LCEC) for the quantitation of norepinephrine and serotonin, with Bielshowski and Giemsa stains for additional information about neural supply, and with acetylcholinesterase histochemistry for the localization of this hydrolytic enzyme. Fluorescent plexuses entered the serosal surface of the appendix associated with blood vessels, traveled longitudinally inside the muscularis interna, mainly in association with blood vessels but adjacent to enteric smooth muscle, and branched into long, linear, varicose plexuses that ran inward in a radial orientation towards the lumen in the internodular septa. As these fibers approached the interdomal regions near the epithelial surface, they passed through thymus-dependent cell zones, and arborized extensively throughout the interdomal region. A high density of varicosities was found in the subepithelial region where immunoglobulin-secreting plasma cells are found. These varicosities were sparse at 21 days of age, but were increased in number at 42 days of age. They were even further increased in number and density in adults. These fibers were further identified with a Bielshowski silver stain, and also demonstrated acetylcholinesterase activity. The noradrenergic varicosities in the interdomal regions of the adult rabbit appendix were closely associated with numerous yellow fluorescent cells of 25-40 microns diameter, which sometimes demonstrated fine varicose processes. The adult rabbit appendix contained a moderate concentration of norepinephrine (163.0 +/- 22.9 ng/g wet weight) and a very high concentration of serotonin (3981 +/- 283 ng/g wet weight). Levels in neonates were considerably lower, suggesting that the yellow fluorescent cells may contain serotonin. Acetylcholinesterase was associated with neural fibers and with non-neural regions of the lymph nodules and the domes, perhaps playing a protective role for these regions of the cellular immune system. The rabbit appendix is a well organized region of lymphoid tissue with specific zones of noradrenergic innervation and possible "paraneuronal" activity, with a readily accessible lumen for the isolation and collections of secretions. We propose this model as an excellent structure for further exploration of interactions between the nervous and immune systems.