The freeze-fracture appearance of the nexus was compared in the smooth muscle of guinea pig sphincter pupillac, portal vein, pulmonary artery, taenia coli, uretzr, and vas diferens, mouse vas deferens, chicken gizzard and anterior mesenteric artery, and toad stomach. Nexuses are particularly numerous in the guinea pig sphincter pupillae; they are usually oval and their average area is 0.15 mum2, although some as large as 0.6 mum2 were seen. Small aggregations of particles were observed which would not be recognizable as nexuses in thin section. What constitutes the minimum size of a nexus is discussed. It is estimated that the number of nexuses per cell in this preparation is of the order of tens rather than hundreds. All nexuses examined had 6-9-nm particles in the PF face, with corresponding 3-4-nm pits on the EF face forming a polygonal tending towards a hexagonal lattice. The nexuses are arranged in rows parallel to the main axis of the cell, usually alternating with longitudinal rows of plasmalemmal vesicles. Many nexuses in the guinea pig sphincter pupillae, chicken gizzard, and toad stomach show a close relationship with sarcoplasmic reticulum. The possibility that this may have some role in current flow across this specialized junction is discussed.

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