The distribution of gamma-aminobutyric acid (GABA)-like and glutamate decarboxylase (GAD)-like immunoreactivity was studied in the cervical sympathetic ganglion complex of rats, including the intermediate and inferior cervical ganglia and the uppermost thoracic ganglion. GABA-positive axons may enter the ganglion complex via its caudal end. Others apparently arise from small GABA-positive cell bodies which are scattered among principal neurons, within clusters of SIF cells and in bundles of GABA-negative axons. The majority of these cells is located in the lower half of the ganglion complex. Principal neurons did not react with antibodies against GABA or GAD. An unevenly distributed mesh-work of GABA-immunoreactive axons was seen in each of the ganglia. Immunoreactive axons formed numerous varicosities. Some of them were aggregated in a basket-like form around a subpopulation of GABA-negative principal ganglion cell bodies. Electron-microscopic immunocytochemistry revealed that GABA-positive nerve fibers establish asymmetric synaptic junctions with dendritic and somatic spines of principal neurons, whereas postsynaptic densities are inconspicuous or absent on dendritic shafts and somata. The results suggest that in the cervical sympathetic ganglion complex principal neurons are not GABAergic, but are innervated by axons which react with both antibodies against GAD and/or GABA antibodies and originate from a subpopulation of small neurons.