The postnatal development of neuropeptide Y- and calcitonin gene-related peptide-immunoreactive (NPY-IR and CGRP-IR) nerve fibers in the rat urinary bladder was investigated using whole-mount preparations and cryostat sections. In newborn and 3-day-old rats, many NPY-IR nerve fibers were observed in the subserous and muscle layers. Many NPY-IR nerve cell bodies clustered at branching points of the subserous nerve bundles. Within 4 weeks after birth, these cell bodies drastically decreased in number and spread along the bundles, although the number of NPY-IR nerve fibers increased moderately. In contrast, CGRP-IR nerve fibers in newborn and 3-day-old rats were less developed, and no CGRP-IR nerve cell body was observed in any rat. However, CGRP-IR nerve fiber distribution in the urinary tissues conspicuously increased within 4 weeks after birth. Especially, an increase of the infraepithelial fibers showing a meshwork appearance was prominent in the fundus and corpus of the bladder. The infra- and intraepithelial CGRP-IR nerve meshwork of the ventral wall was more dense than that of the trigone. At 4 weeks, NPY-IR and CGRP-IR nerves were similar to those of the adult rat (8-12 weeks old). The present study suggests a correlation between the development of the peripheral nervous system in the urinary bladder and maturation of micturition behavior in the rat.