Pseudorabies virus was injected into the wall of the urinary bladder in 2- and 12-day-old rats in order to examine developmental changes in the central neural pathways controlling micturition. Forty-eight hours after virus injection, virus-labeled neurons were identified in the lumbosacral spinal cord but not in the brain. Sixty to seventy-two hours after virus injection in both 2- and 12-day-old rats, infected neurons were detected in Barrington's nucleus, nucleus paragigantocellularis, nucleus reticularis gigantocellularis, A5 area, nucleus raphe obscurus, locus subcoeruleus, periaquaductal gray, red nucleus, paraventricular nucleus and cerebral cortex. These results in neonatal rats are similar to those reported in adult rats. Thus, it is likely that the supraspinal neural circuitry which underlies micturition in the adult animal is already organized in the neonatal rat during the early postnatal period even though the supraspinal micturition reflex pathway does not become functional until the third postnatal week.