The distribution of substance P (SP)-immunoreactive fibers and terminals in the rat spinal cord during postnatal development was investigated by peroxidase-antipe roxidase (PAP) immunocytochemistry, and the effect of neonatal androgen exposure on their distribution was studied. In the male rats, a dense network of SP-immunoreactive fibers and terminals, and terminals was detected in the ventral ... [Show full abstract] column of the rostral lumber segments with a density and extent different from the other segmental levels during the postnatal period between neonates and adults. These fibers and terminals were accumulated within and around the nucleus centromedialis lumbaris (CM) (15) of the L1 and L2 segments (L1-2). However, in the female rats, SP-immunoreactive fibers and terminals in their densities during the postnatal period. Neonatal androgen exposure resulted in an increase in the SP-immunoreactivity in the ventral column of the rostral lumber segments of the female rats. These results indicate that the ventral horn of the L1-2 Shows sexual dimorphism in the distributional pattern of SP-immunoreactive fibers and terminals, and suggest that postnatal development of the densely distributed SP-immunoreactivity in these areas is partly due to androgenic rugulation to the central nervous system.