Immunocytochemistry using antisera specific for the G-protein ␣ subunits G ␣i , G ␣q , and G ␣s revealed similar patterns of immunoreactivity in the lobster brain. Immunoreactivity was strongest in neuropil, especially the olfactory and accessory lobes, and was characterized by bundles of fine threads leading to dense concentrations of punctate staining in the glomeruli. This may reflect the concentration of G-protein ␣ subunits at synapses. The major differences between the antisera were distinct patterns of staining intensity in subregions of glomeruli of the olfactory and accessory lobes. This result is potentially correlated with previous evidence that these subregions are neurochemically distinct. Neuronal cell bodies contained moderate levels of immunoreactivity at the plasma membrane and faint staining in the cytoplasm. The olfactory globular tract was moderately immunoreactive, but other fiber tracts were weakly immunoreactive. Immunoreactivity in the deutocerebral commissure consisted of small oval cell bodies and strands that formed a reticulated pattern, suggestive of glia. Photoaffinity labelling by using an analog of GTP demonstrated that histamine activated G ␣i in brain homogenates. Further evidence of G-protein activation was obtained by showing that stimulation with a mixture of neuroactive substances increased the amount of phospholipase C- associated with membranes, G ␣q , and G  . The lobster brain, especially in its neuropil regions, is richly endowed with neuromodulatory biochemical pathways involving G ␣i , G ␣q , and G ␣s .