Immunocytochemical studies of the rat carotid body during postnatal development revealed neuropeptide Y (NPY), tyrosine hydroxylase (TH), and dopamine beta-hydroxylase (DBH) immunoreactivities. In adult rats (at postnatal week 10), NPY and DBH immunoreactivities were shown in a few small chief cells (cell number/section shown as mean +/- SD: NPY 3.4+/-2.6, DBH 3.2+/-2.3), in large ganglion cells, and in numerous varicose nerve fibers of the carotid body. TH immunoreactivity was found in almost all chief cells, in a few ganglion cells, and in numerous varicose nerve fibers in the carotid body. By using the double-immunostaining technique, most NPY-immunoreactive chief cells, ganglion cells, and nerve fibers exhibited DBH immunoreactivity. The NPY- and DBH-immunoreactive chief cells in the rat carotid body were numerous from birth (NPY 93.8+/-14.9, DBH 89.7+/-12.3) to postnatal week 1 (NPY 65+/-14.5, DBH 61.6+/-11.3), but decreased quickly from postnatal week 2 (NPY 6.1+/-3.5, DBH 3.6+/-2.8) onwards. A few NPY- and DBH-immunoreactive ganglion cells were found in the periphery or in the center of the rat carotid body during postnatal development. TH immunoreactivity was observed in almost all chief cells and in a few ganglion cells in all developmental stages. NPY- and DBH-immunoreactive nerve fibers were very scarce in the carotid body from birth to postnatal week 1, began to increase gradually after postnatal week 2, and reached the adult level by postnatal week 5. The present study suggests that the expression of NPY and noradrenaline in chief cells and in the nerve fibers of the rat carotid body may be regulated during postnatal development.