The paraventricular nucleus of the hypothalamus (PVN) modulates vagal digestive motor functions via oxytocinergic projections to the nucleus of the solitary tract (NST) and dorsal motor nucleus of the vagus (DMV) in adult rats. Little is known regarding the structural or functional maturation of these projections. The present study examines the postnatal development of immunocytochemically identified oxytocinergic fibers in gastric subregions of the medial NST-DMV. For this purpose, a monoclonal antibody (PS36) that recognizes both oxytocin (OT)-neurophysin and its prohormone was used to identify oxytocinergic fibers. PS36-positive fibers already were present within the NST-DMV in rats on the day of birth. Retrograde transport of cholera toxin neural tracer from the NST-DMV in newborn rats confirmed that PVN neurons were the sole source of these oxytocinergic fibers. The cumulative length of PS36-positive fibers in sampled subregions of the medial NST and DMV increased approximately 23-fold and 94-fold, respectively, between birth and adulthood. The observed postnatal increases in PS36 immunolabeling could reflect increased delivery of immunoreactive antigen from hypothalamic perikarya to distal axons and/or increasing oxytocinergic innervation of the NST-DMV. Additional work will be needed to address these questions and to determine the time course during which central oxytocinergic pathways become mature in their ability to influence vagally mediated digestive functions.