Studies were performed in the opossum to evaluate the morphological characteristics of the lower esophageal sphincter. The sphincter and the esophageal body were identified manometrically and fixed in situ by perfusion with aldehyde fixative. Light microscopy revealed that: (1) longitudinal muscle layers of the sphincter and the esophageal body were similar in thickness and compactness, and (2) circular muscle of the sphincter was thicker and was composed of muscle fasciculi with abundant intervening connective tissue as compared to the compact muscle fasciculi of the circular muscle of the esophageal body. Electron microscopy showed the circular muscle fibers of the sphincter to have irregular protuberances from their surfaces, whereas the circular muscle fibers of the esophageal body possessed smooth surfaces. Several types of junctional complexes between adjacent muscle fibers were observed; however, there was no difference in their distribution in the sphincter or esophageal body. The majority of the nerve varicosities contained a mixture of agranular (350 to 450 A) and large dense core vesicles (800 to 1600 A); varicosities containing small granular vesicles were not found in any area of the esophagus. When varicosities were cut along the longitudinal axis of the axon, they often showed grouping of similar vesicle types in different areas along the axon varicosities. There was no difference in the type of varicosities found in the sphincter or the esophageal body. These studies show that: (1) circular muscle of the sphincter can be morphologically distinguished from that of the esophageal body; (2) there is no difference in the morphology of the nerve terminals or the vesicle types in the two areas; and (3) classification of the varicosities based upon the predominance of the vesicle types may be artifactual because of sampling error.