In the present immunohistochemical study the distribution of nitric oxide synthase (NOS) was studied in various autonomic ganglia and in related peripheral tissues of the rat. For comparison some other neuronal markers including acetylcholinesterase and tyrosine hydroxylase as well as several neuropeptides were analysed on adjacent or the same sections. The distribution of NOS-like immunoreactivity (LI) and of these other markers has been semiquantitatively summarized. In some parasympathetic ganglia such as the sphenopalatine and submandibular ganglia NOS-LI was present in most ganglion cells, at least partly coexisting with peptide histidine isoleucine (PHI), vasoactive intestinal polypeptide (VIP) and neuropeptide tyrosine (NPY). In the pelvic ganglia a comparatively smaller proportion of neurons was NOS-positive and they often contained VIP-LI and less frequently NPY-LI. In the tissues innervated by these ganglia, such as nasal mucosa and salivary glands, NOS-positive fibers were observed around blood vessels and within the glandular parenchyma, although generally less abundant than VIP/PHI nerves, while in the uterus, vas deferens and penis a more close correlation was seen. NOS-positive fibers were also widely distributed in other tissues. In the sympathetic ganglia NOS-LI was mainly present in dense fiber networks, which disappeared after transection of the sympathetic trunc central to the ganglion. Since many cell bodies in the sympathetic lateral column of the spinal cord also were NOS-positive, it is likely that the majority of preganglionic fibers innervating sympathetic ganglia are NOS-positive. VIP-positive cells in stellate ganglia did not contain NOS-LI. The present results suggest that NO may be a messenger molecule both in parasympathetic postganglionic neurons and in preganglionic sympathetic neurons.