Indentations (hillocks and dimples) in the terminal cisternae of mammalian and amphibian skeletal muscle fibers were studied using freeze-fracture and serial thin-section techniques. The structures were seen in all muscles and had a regular separation from each other and from the T-tubule. Indentations were smaller than fenestrations and formed concavities in, but not macromolecular pores through, the terminal cisternae. The average numbers of indentations in rat muscles (measured along the length of the terminal cisternae, within 150 nm of the triadic junction) varied from 0.9 per micrometer in soleus fibers to 9.6 per micrometer in posterior cricoarytenoid fibers. The average numbers in amphibian sartorius fibers varied from 1.6 to 3.6 per micrometer in muscles from different species. The regular alignment of the indentations along the triad, as well as a close correlation between their numbers and the contractile properties of the muscle, suggest that they function in contractile activation and may represent sites of calcium release from the terminal cisternae.