The ultrastructure of the longitudinal and circular muscle cells of the guinea pig stomach, known to display different contractile responses, was compared. The longitudinal muscle layer consisted of about 20 layers of smooth muscle cells and the extracellular space occupied about 12.1% of the cross sectional area. The circular muscle layer consisted of closely packed muscle bundles arranged side by side. The extracellular space within the bundle represented about 4.4% of the cross sectional area. Nexuses were consistently found in the circular muscle layer but could not be found in the longitudinal muscle layer. Numbers of both mitochondria and microtubules per unit area of smooth muscle cell were larger in the longitudinal than in the circular muscle. The cell area occupied by the sarcoplasmic reticulum was about 4.7% in the longitudinal muscle cell, twice as much as in the circular muscle cell (2.3%). Numbers of caveolae per micrometer of the cell perimeter were almost the same in both tissues. There were approximately 25 and 50 thick filaments per 0.5 μm2 of cytoplasmic area in the longitudinal and circular muscle cell, respectively. A lower pH fixative (cacodylate, pH 6.6) gave a better contrast of specimens than other fixatives used, and an organic buffered (PIPES) fixative led to a more regular arrangement of myofilaments. But the characteristic distribution of the thick filaments between both muscles was not different among the specimens fixed with these fixatives
