The purpose of the study was to investigate rod-like structures (rods) in the terminal cisternae membrane of freeze fracture replicas of fast- and slow-twitch mammalian muscle. The 9 X 50 nm rods crossed the junctional gap perpendicular to the T-tubule membrane and terminated near indentations. Rods are likely to have a structural basis because (1) grooves were seen in the complimentary membrane leaflet, (2) rods were seen in tissue fixed in 0.5, 5, or 6% glutaraldehyde or 5% acrolein, (3) rods were seen in tissue fractured over a range of temperatures from -40 to -196 degrees C, (4) the number of rods was correlated with the contractile properties of fibers, and (5) the density of rods increased when fibers were depolarized before fixation. The rods are in a unique location that would allow them to participate in excitation-contraction coupling, perhaps by transmitting an electrical signal from the T-tubule membrane to calcium release sites in the terminal cisternae.