The anatomical localization of gamma-aminobutyric acid type A (GABA-A) receptor sites in the rat superior cervical ganglion was studied using combined radioreceptor binding and autoradiographic techniques. 3H-Muscimol was used as a ligand of GABA-A receptor sites. The binding was consistent with the labelling of GABA-A sites. The dissociation constant value was 6.4 nmol/l, and the maximum density of binding sites was 146 +/- 7.8 fmol/mg tissue. Light microscope autoradiography revealed the accumulation of 3H-muscimol mainly in superior portions of the ganglion. Binding sites are located primarily in the neuropil rather than within ganglionic neurons. It is probable that the sites revealed by autoradiography are involved in the inhibition of acetylcholine release from ganglionic neurons.