Nitric oxide synthesizing neurons of the locust CNS have been identified by NADPH-diaphorase staining. However, the conventional transmitters of these neurons are unknown. Here we use double labelling for NADPH-diaphorase and GABA-immunofluorescence on sections of the brain to investigate a potential coexpression of both markers. The antennal lobe is innervated by a cluster of about 45-50 NADPH-diaphorase positive local interneurons which express GABA-immunofluorescence. The mushroom bodies are a higher order olfactory center which receive an extrinsic innervation from GABA-immunoreactive and NADPH-diaphorase positive fiber systems. Each optic lobe contains about 4500 GABA-immunoreactive cell bodies. In the visual system, identifiable GABA-immunoreactive neurons arborize in the external plexiform layer of the lamina, in several strata of the medulla, and in the lobula complex. A survey of all NADPH-diaphorase positive cell groups detected a colocalization of GABA-immunoreactivity in a small subpopulation of somata along the anterior rim of the medulla. These cytochemical findings suggest that nitric oxide may be a characteristic cotransmitter of GABAergic circuits of the antennal lobe, while in mushroom bodies and the visual system the majority of nitric oxide and GABA releasing neurons are distinct populations.