The distribution and origin of the nerve fibers innervating the rat thyroid were studied by immunocytochemistry in experiments involving retrograde tracing and denervations. Nerve fibers containing noradrenaline (NA), neuropeptide Y (NPY), vasoactive intestinal peptide (VIP), galanin (GAL), substance P (SP), and calcitonin gene-related peptide (CGRP) occurred around blood vessels and follicles. Many of these transmitter candidates coexisted with each other in different combinations in different subpopulations of neurones. Injection of the retrograde tracer True Blue into the thyroid gland labelled cell bodies in the thyroid ganglion, the laryngeal ganglion, the superior cervical ganglion, the jugular-nodose ganglionic complex, the dorsal root ganglia (C2-C5) and the trigeminal ganglion. Judging from the number of labelled nerve cell bodies, the superior cervical ganglion and the thyroid ganglion contribute most to the thyroid innervation, while the laryngeal ganglion and the trigeminal ganglion contribute least. Results from denervation and tracer experiments indicate that all NA-containing and the majority of NPY-containing nerve fibers in the thyroid derive from the superior cervical ganglion. All VIP- and a minor population of NPY- and GAL-containing fibers in the thyroid gland originate in the thyroid ganglion. Nerve fibers containing GAL, SP and/or CGRP emanate from the jugular, cervical dorsal root and/or trigeminal ganglia. Together the findings indicate that several ganglia project to the thyroid and that many neuropeptides may be involved in the control of thyroid activity.