The mushroom body (MB) of the insect brain is critical to associative memory formation. Intrinsic neurons within the MB (called Kenyon cells, KCs) receive sensory signals from input neurons in the calyces. The calyces of the cockroach MB receive branches of four giant neurons (calycal giants, CGs) which exhibit gamma-aminobutyric acid (GABA)-like immunoreactivity. Here we examined the CGs by intracellular recording and staining. The CGs have dendritic arborizations in the lateral horn (lateral protocerebral lobe) and the neuropil anterior to the alpha and beta lobes (output neuropils of the MB); their terminal arborizations cover the entire calyces. The CGs exhibit a spontaneous and rhythmic burst of spikes, which are suppressed by olfactory, visual, tactile or air current stimulation. The CGs may facilitate, by disinhibition, the acquisition of sensory signals by the KCs when the insect is aroused by sensory stimuli.