The morphology, distribution, and configuration of coiled perimysial fibers of rat heart papillary muscle were studied. Methods included bright-field light microscopy of silver-stained sections, scanning and transmission electron microscopy, and differential interference contrast light microscopy of unfixed and unstained specimens. Coiled fibers, elliptical in cross section, are arranged in a branched network that diverges from the muscle-tendon junction and is continuous throughout the length of the muscle and into the ventricle wall. Most fibers range in diameter from less than 1 micron to 10 micron and are parallel with the long axis of the muscle, although branching is common and oblique orientations are seen. Several myocytes are associated with each coiled perimysial fiber. Constituent fibrils (diameter, 40-50 nm) occur in bundles twisted within the fiber. Small satellite elastic fibers are parallel to the collagen fiber axes. Stereo analysis of the coiled perimysial fibers reveals helical configurations, as opposed to planar waviness, that become less convoluted or even straighten as the resting muscle is stretched. Calculations based on cross-sectional areas of fibers, changes in fiber configurations, and tensile moduli reported for collagen fibers of other tissues show that the potential tensile strength of the network of coiled perimysial fibers is sufficient to contribute significantly to the mechanical properties of papillary muscle. Detailed evaluations of possible roles of the coiled perimysial collagen fiber system as a function of passive stretch and contraction in ventricular wall, as well as in papillary muscle, warrant further study.