The morphologic changes in the walls of hollow viscera caused by contraction and relaxation of smooth muscle cells were studied from autopsy and surgical specimens. The specimens studied included: esophageal spasm (corkscrew and nutcracker esophagus), contraction of the lower esophageal sphincter with marked esophageal dilatation, gaseous distension of the stomach, contraction of the gastric pylorus, bladder and anal sphincter, physiological segmental constriction of the small and large intestines, constriction and distension of the gallbladder, urinary bladder and bronchi, and postpartum contraction of the uterus. In contrast to distension, the constriction of hollow viscera shows marked reduction of the external circumference and diameter with thickening of the wall, contraction of smooth muscle cells, thickening of muscle bundles, remodeling of wall structure, and narrowing or obliteration of the lumen. Morphologic evidence of contraction of smooth muscle cells is demonstrated by varying degrees of typical lengthwise shortening of the cells and squeezing and folding of the nuclei depending on the degree of cytoplasmic contraction of the smooth muscle cells. Using these same classic morphologic signs, we have attempted to study constriction and distension of arteries and arterioles. We can demonstrate contraction of smooth muscle cells and remodeling of arterial and arteriolar walls in patients with spastic coronary artery thrombosis, cocaine-induced coronary artery thrombosis, acute constriction of mesenteric arteries with lacerations of arterial wall, and dissecting hemorrhages induced by large doses of intravenous infusion of vasoconstrictors for hemorrhagic shock, and in patients with sustained, accelerated, or malignant hypertension.