Abundant, highly organized, rod-shaped particles have been found in skeletal-muscle nuclei of two patients with adult-onset rod disease. They were usually single in affected nuclei. Like myofibrillar rods, the nuclear rods consisted of bundles of long, parallel, apparently cross-linked filaments. On longitudinal section the rods had an axial periodicity and in transverse section a wire-mesh appearance. Average periodicities were 189A (nuclear) and 178A (myofibrillar) axially, 180A (both) transversely between the longitudinal rod-filaments on longitudinal-section and cross-section of the rods. Minor differences were that the nuclear rods were slightly lighter stained after osmium-uranyl acetate-lead citrate, lacked thin filaments protruding from their ends, and often were broader. It is proposed that nuclear rods may have a contractile-protein composition and pathokinesis similar to that of myofibrillar rods. Their formation may reflect an epitomization of a newly recognized general biological capability of exogenous, or perhaps endogeneous, nuclear protein alteration.