An electron microscope study has been carried out on rat psoas muscle, during the early postnatal stages of development. Among the several subcellular components, the sarcotubular system undergoes the most striking modifications during this period. In muscle fibers of the newborn rat, junctional contacts between the T system and the SR are sparse and are, mostly, longitudinally or obliquely oriented. The T tubules do not penetrate deeply into the muscle cell, as indicated by the predominantly peripheral location of the triads and the persistence, at these stages of development, of a highly branched subsarcolemmal system of tubules. Diadic associations of junctional SR elements with the plasma membrane are also occasionally observed. The early SR elaborations incompletely delineate the myofibrils, at both the A- and I-band level. Longitudinal sections show irregularly oriented SR tubules, running continuously over successive sarcomeres. Flattened junctional cisterns filled with granular material are sparse and laterally interconnected, at circumscribed sites, with the SR tubules. Between 1 and 2 wk postpartum, transversal triadic contacts are extensively established, at the A-I band level, and the SR network differentiates into two portions in register with the A and I band, respectively. At 10-15 days after birth, the SR provides a transversely continuous double sheet around the myofibrils at the I-band level, whereas it forms a single discontinuous layer at the A-band level. The relationship that these morphological modifications of the sarcotubular system may bear to previously described biochemical and physiological changes of rat muscle fibers after birth is discussed.