Abstract
Sections of adult mammalian cardiac muscles fixed at room temperature reveal numerous microtubules (24--28 nm in diameter) both near the nucleus and in the extra-myofibrillar space. Microtubules encircle the nucleus, are associated with the myofibrils in a helical arrangement, and form a network that runs transversely at the level of the I band and axially between the myofibrils. Microtubules are more numerous in muscle cells than previously recognized and may perform more than cytoskeletal function.