The postnatal (0-24 days) development of substance P (SP)-, calcitonin gene-related peptide (CGRP)- and neuropeptide Y(NPY)-like immunoreactive (LI) nerves in the rat temporomandibular joint (TMJ) was investigated immunohistochemically. Immediately after birth, SP- or CGRP-LI nerves were observed in most disc attachments. A few NPY-LI nerves were observed around the large blood vessels in the joint capsule. From days 3 to 6, the SP- or CGRP-LI nerves were first found close to the anterior, lateral, medial (third day) or posterior (sixth day) peripheral portion of the disc. The synovial cells (type A and B) first appeared at the anterior peripheral portion of the disc (sixth day), and then at the posterior, lateral and medial portions (seventh day). NPY-LI nerves were found around the blood vessels at the disc attachment on the sixth day, and then entered into the peripheral portion of the disc from days 10 to 14. At 14 days a few NPY-LI nerves were first found close to the blood vessels in the sublining layer of the synovial membrane. From days 18 to 24, a few NPY-LI nerves were located in the superficial layer of the synovial membrane. The central portion of the disc did not contain any nerves from days 0 to 24. Thus SP- or CGRP-LI sensory nerves are shown to innervate the rat TMJ at an earlier age than NPY-LI sympathetic nerves, which may modulate the regulation of blood flow in the joint capsule, disc and synovial membrane. However, it is considered that the disc itself does not contribute to the transportation of the afferent sensory information. Furthermore, from the fact that SP- or CGRP-LI nerves were found earlier than the appearance of the synovial cells, it is suggested that these nerves may be associated with the growth and proliferation of synovial cells.