Nitric oxide synthase-immunoreactivity (NOS-IR) was detected in strands of nerve fibers entering the rat superior cervical ganglia (SCG) and in nerve fibers forming a plexus beneath the capsule of adrenal glands. Within the SCG, varicose NOS-IR fibers encircled virtually all postganglionic neurons and small diameter cells, presumably small intensely fluorescent (SIF) cells. Perikarya of SIF cells exhibited strong NOS-IR, whereas the level appeared to be low in postganglionic neurons. Decentralization of the SCG for 4-6 days markedly reduced the number as well as the intensity of NOS-IR fibers without causing a detectable change of NOS-IR in the postganglionic neurons and SIF cells. Beneath the adrenal capsule, bundles of NOS-IR fibers bifurcated and made a sharp turn to reach the adrenal medulla. Chromaffin cells, which themselves exhibited fairly strong NOS-IR, appeared to be surrounded by NOS-IR fibers. The result shows that NOS-IR is present in pre- and post-synaptic elements of the sympathetic ganglia and adrenal medulla, representing a complex system that may regulate the activity of ganglionic neurons and chromaffin cells via a number of sites of action.