Nitric oxide is generated by a Ca2+/calmodulin-stimulated nitric oxide synthase and activates soluble guanylyl cyclase. Using NADPH diaphorase (NADPHd) staining as a marker for the enzyme nitric oxide synthase and an antiserum against cGMP, we investigated the cellular organization of nitric oxide donor and target cells in olfactory pathways of the brain of the locust (Schistocerca gregaria). A small subset of neuronal and glial cells expressed cGMP immunoreactivity after incubation of tissue in a nitric oxide donor. Nitric oxide-induced increases in cGMP immunoreactivity were quantified in a tissue preparation of the antennal lobe and in primary mushroom body cell cultures. The mushroom body neuropil is a potential target of a transcellular nitric oxide/ cGMP messenger system since it is innervated by extrinsic NADPHd-positive neurons. The mushroom body-intrinsic Kenyon cells do not stain for NADPHd but can be induced to express cGMP immunoreactivity. The colocalization of NADPHd and cGMP immunoreactivity in a cluster of interneurons of the antennal lobe, the principal olfactory neuropil of the insect brain, suggests a role of the nitric oxide/cGMP system in olfactory sensory processing. Colocalization of NADPHd staining and cGMP immunoreactivity was also found in certain glial cells. The cellular organization of the nitric oxide/cGMP system in neurons and glia raises the possibility that nitric oxide acts not only as an intercellular but also as an intracellular messenger molecule in the insect brain.